[PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Frej Drejhammar]

The cx2388x family of broadcast decoders all have features not enabled
by the standard cx88 driver. This revised patch series adds controls
allowing the chroma AGC and the color killer to be enabled (Version 1
of the series used module parameters). By default both features are
disabled as in previous versions of the driver.

The Chroma AGC and the color killer is sometimes needed when using
signal sources of less than optimal quality.

The patches applies cleanly to 7370:11fdae6654e8 of
http://linuxtv.org/hg/v4l-dvb/ and have been tested an a HVR-1300. The
patches should be applied in order.

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[PATCH 1 of 2] cx88: Add user control for chroma AGC

[Frej Drejhammar]

4 files changed, 42 insertions(+), 3 deletions(-)
linux/drivers/media/video/cx88/cx88-blackbird.c |    1 
linux/drivers/media/video/cx88/cx88-core.c      |    6 +++
linux/drivers/media/video/cx88/cx88-video.c     |   37 +++++++++++++++++++++--
linux/drivers/media/video/cx88/cx88.h           |    1 


# HG changeset patch
# User "Frej Drejhammar <frej.drejhammar@gmail.com>"
# Date 1205668069 -3600
# Node ID ac2c307ae12abeb35b59dc0bb1aef95ac6f51798
# Parent  11fdae6654e804fdd000b0c3c9f3a4a7344ffd69
cx88: Add user control for chroma AGC

From: "Frej Drejhammar <frej.drejhammar@gmail.com>"

The cx2388x family has support for chroma AGC. This patch adds a user
control, "Chroma AGC", controlling it. By default chroma AGC is
disabled, as in previous versions of the driver.

Signed-off-by: "Frej Drejhammar <frej.drejhammar@gmail.com>"

diff -r 11fdae6654e8 -r ac2c307ae12a linux/drivers/media/video/cx88/cx88-blackbird.c
--- a/linux/drivers/media/video/cx88/cx88-blackbird.c	Fri Mar 14 00:38:24 2008 -0300
+++ b/linux/drivers/media/video/cx88/cx88-blackbird.c	Sun Mar 16 12:47:49 2008 +0100
@@ -701,6 +701,7 @@ static const u32 *ctrl_classes[] = {
 static const u32 *ctrl_classes[] = {
 	cx88_user_ctrls,
 	cx2341x_mpeg_ctrls,
+	cx88_priv_ctrls,
 	NULL
 };
 
diff -r 11fdae6654e8 -r ac2c307ae12a linux/drivers/media/video/cx88/cx88-core.c
--- a/linux/drivers/media/video/cx88/cx88-core.c	Fri Mar 14 00:38:24 2008 -0300
+++ b/linux/drivers/media/video/cx88/cx88-core.c	Sun Mar 16 12:47:49 2008 +0100
@@ -958,7 +958,11 @@ int cx88_set_tvnorm(struct cx88_core *co
 
 	dprintk(1,"set_tvnorm: MO_INPUT_FORMAT  0x%08x [old=0x%08x]\n",
 		cxiformat, cx_read(MO_INPUT_FORMAT) & 0x0f);
-	cx_andor(MO_INPUT_FORMAT, 0xf, cxiformat);
+	/* Chroma AGC must be disabled if SECAM is used */
+	if (norm & V4L2_STD_SECAM)
+		cx_andor(MO_INPUT_FORMAT, 0x40f, cxiformat);
+	else
+		cx_andor(MO_INPUT_FORMAT, 0xf, cxiformat);
 
 #if 1
 	// FIXME: as-is from DScaler
diff -r 11fdae6654e8 -r ac2c307ae12a linux/drivers/media/video/cx88/cx88-video.c
--- a/linux/drivers/media/video/cx88/cx88-video.c	Fri Mar 14 00:38:24 2008 -0300
+++ b/linux/drivers/media/video/cx88/cx88-video.c	Sun Mar 16 12:47:49 2008 +0100
@@ -177,6 +177,11 @@ static struct cx8800_fmt* format_by_four
 
 /* ------------------------------------------------------------------- */
 
+/* Private controls for cx2388x */
+#define CX88_CID_PRIVATE_CLASS V4L2_CID_PRIVATE_BASE
+#define CX88_CID_CHROMA_AGC   (V4L2_CID_PRIVATE_BASE + 0)
+#define CX88_CID_LAST_PRIVATE (V4L2_CID_PRIVATE_BASE + 1)
+
 static const struct v4l2_queryctrl no_ctl = {
 	.name  = "42",
 	.flags = V4L2_CTRL_FLAG_DISABLED,
@@ -244,6 +249,18 @@ static struct cx88_ctrl cx8800_ctls[] = 
 		.mask                  = 0x00ff,
 		.shift                 = 0,
 	},{
+		.v = {
+			.id            = CX88_CID_CHROMA_AGC,
+			.name          = "Chroma AGC",
+			.minimum       = 0,
+			.maximum       = 1,
+			.default_value = 0x0,
+			.type          = V4L2_CTRL_TYPE_BOOLEAN,
+		},
+		.reg                   = MO_INPUT_FORMAT,
+		.mask                  = 1 << 10,
+		.shift                 = 10,
+	}, {
 	/* --- audio --- */
 		.v = {
 			.id            = V4L2_CID_AUDIO_MUTE,
@@ -302,8 +319,16 @@ const u32 cx88_user_ctrls[] = {
 };
 EXPORT_SYMBOL(cx88_user_ctrls);
 
+const u32 cx88_priv_ctrls[] = {
+	CX88_CID_PRIVATE_CLASS,
+	CX88_CID_CHROMA_AGC,
+	0
+};
+EXPORT_SYMBOL(cx88_priv_ctrls);
+
 static const u32 *ctrl_classes[] = {
 	cx88_user_ctrls,
+	cx88_priv_ctrls,
 	NULL
 };
 
@@ -311,8 +336,10 @@ int cx8800_ctrl_query(struct v4l2_queryc
 {
 	int i;
 
-	if (qctrl->id < V4L2_CID_BASE ||
-	    qctrl->id >= V4L2_CID_LASTP1)
+	if ((qctrl->id < V4L2_CID_BASE ||
+	     qctrl->id >= V4L2_CID_LASTP1) &&
+	    (qctrl->id < V4L2_CID_PRIVATE_BASE ||
+	     qctrl->id >= CX88_CID_LAST_PRIVATE))
 		return -EINVAL;
 	for (i = 0; i < CX8800_CTLS; i++)
 		if (cx8800_ctls[i].v.id == qctrl->id)
@@ -1225,6 +1252,12 @@ int cx88_set_control(struct cx88_core *c
 		}
 		mask=0xffff;
 		break;
+	case CX88_CID_CHROMA_AGC:
+		/* Do not allow chroma AGC to be enabled for SECAM */
+		value = ((ctl->value - c->off) << c->shift) & c->mask;
+		if (core->tvnorm & V4L2_STD_SECAM && value)
+			return -EINVAL;
+		break;
 	default:
 		value = ((ctl->value - c->off) << c->shift) & c->mask;
 		break;
diff -r 11fdae6654e8 -r ac2c307ae12a linux/drivers/media/video/cx88/cx88.h
--- a/linux/drivers/media/video/cx88/cx88.h	Fri Mar 14 00:38:24 2008 -0300
+++ b/linux/drivers/media/video/cx88/cx88.h	Sun Mar 16 12:47:49 2008 +0100
@@ -681,6 +681,7 @@ void cx8802_cancel_buffers(struct cx8802
 /* ----------------------------------------------------------- */
 /* cx88-video.c*/
 extern const u32 cx88_user_ctrls[];
+extern const u32 cx88_priv_ctrls[];
 extern int cx8800_ctrl_query(struct v4l2_queryctrl *qctrl);
 int cx88_enum_input (struct cx88_core  *core,struct v4l2_input *i);
 int cx88_set_freq (struct cx88_core  *core,struct v4l2_frequency *f);

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[PATCH 2 of 2] cx88: Add user control for color killer

[Frej Drejhammar]

1 file changed, 15 insertions(+), 1 deletion(-)
linux/drivers/media/video/cx88/cx88-video.c |   16 +++++++++++++++-


# HG changeset patch
# User "Frej Drejhammar <frej.drejhammar@gmail.com>"
# Date 1205668146 -3600
# Node ID 8843d200ee091ef49579555ef6ae4de031986342
# Parent  ac2c307ae12abeb35b59dc0bb1aef95ac6f51798
cx88: Add user control for color killer

From: "Frej Drejhammar <frej.drejhammar@gmail.com>"

The cx2388x family has a color killer. This patch adds a user control,
"Color killer", controlling it. By default the color killer is
disabled, as in previous versions of the driver.

Signed-off-by: "Frej Drejhammar <frej.drejhammar@gmail.com>"

diff -r ac2c307ae12a -r 8843d200ee09 linux/drivers/media/video/cx88/cx88-video.c
--- a/linux/drivers/media/video/cx88/cx88-video.c	Sun Mar 16 12:47:49 2008 +0100
+++ b/linux/drivers/media/video/cx88/cx88-video.c	Sun Mar 16 12:49:06 2008 +0100
@@ -180,7 +180,8 @@ static struct cx8800_fmt* format_by_four
 /* Private controls for cx2388x */
 #define CX88_CID_PRIVATE_CLASS V4L2_CID_PRIVATE_BASE
 #define CX88_CID_CHROMA_AGC   (V4L2_CID_PRIVATE_BASE + 0)
-#define CX88_CID_LAST_PRIVATE (V4L2_CID_PRIVATE_BASE + 1)
+#define CX88_CID_COLOR_KILLER (V4L2_CID_PRIVATE_BASE + 1)
+#define CX88_CID_LAST_PRIVATE (V4L2_CID_PRIVATE_BASE + 2)
 
 static const struct v4l2_queryctrl no_ctl = {
 	.name  = "42",
@@ -261,6 +262,18 @@ static struct cx88_ctrl cx8800_ctls[] = 
 		.mask                  = 1 << 10,
 		.shift                 = 10,
 	}, {
+		.v = {
+			.id            = CX88_CID_COLOR_KILLER,
+			.name          = "Color killer",
+			.minimum       = 0,
+			.maximum       = 1,
+			.default_value = 0x0,
+			.type          = V4L2_CTRL_TYPE_BOOLEAN,
+		},
+		.reg                   = MO_INPUT_FORMAT,
+		.mask                  = 1 << 9,
+		.shift                 = 9,
+	}, {
 	/* --- audio --- */
 		.v = {
 			.id            = V4L2_CID_AUDIO_MUTE,
@@ -322,6 +335,7 @@ const u32 cx88_priv_ctrls[] = {
 const u32 cx88_priv_ctrls[] = {
 	CX88_CID_PRIVATE_CLASS,
 	CX88_CID_CHROMA_AGC,
+	CX88_CID_COLOR_KILLER,
 	0
 };
 EXPORT_SYMBOL(cx88_priv_ctrls);

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Re: [PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Hans Verkuil]

On Sunday 16 March 2008 13:49, Frej Drejhammar wrote:
> The cx2388x family of broadcast decoders all have features not
> enabled by the standard cx88 driver. This revised patch series adds
> controls allowing the chroma AGC and the color killer to be enabled
> (Version 1 of the series used module parameters). By default both
> features are disabled as in previous versions of the driver.
>
> The Chroma AGC and the color killer is sometimes needed when using
> signal sources of less than optimal quality.
>
> The patches applies cleanly to 7370:11fdae6654e8 of
> http://linuxtv.org/hg/v4l-dvb/ and have been tested an a HVR-1300.
> The patches should be applied in order.

Hi Frej,

I have a few comments about this:

1) Should we really expose these settings to the user? I have my doubts 
whether the average user would know what to do with this, and I also 
wonder whether it makes enough of a difference in picture quality. I 
might be wrong, of course. It would help if you could show examples of 
the picture quality with and without these settings. Note that a change 
like 'Chroma AGC must be disabled if SECAM is used' is a bug fix and 
should clearly go in.

2) Chroma AGC and color killer is also present in other chips (cx2584x, 
cx23418, possibly other similar Conexant chips). So if we decide on 
allowing these controls I would prefer making this a standard control, 
rather than a private one.

3) If we decide on allowing these controls, then they must be documented 
in the v4l2 spec.

Personally I think these controls are too low-level, but that's just my 
opinion. Most chips contains a whole array of similar tweaks that you 
can do, and exposing them all is not the way to go.

Regards,

	Hans

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Re: [PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Frej Drejhammar]

Hi Hans,

> 1) Should we really expose these settings to the user? I have my
> doubts whether the average user would know what to do with this, ...

That was my initial take on it, therefore the first version of the
patch just added a module parameter. I reasoned that chroma AGC was
something you just needed to enable once depending on the quality of
your video-source. Then Trent Piepho suggested that the functionality
should really be exposed as controls. I think he has a point, consider
for example living in a place such as southern Germany where you could
receive both German PAL and French SECAM broadcasts. If you then also
used a composite/s-video external video source you would want to be
able to change the setting depending on your input and the channel you
tune to.

> ... and I also wonder whether it makes enough of a difference in
> picture quality.

For me it does, fiddling with the saturation and hue controls I never
managed to get neutral color reproduction. The colors were either
washed out or saturated to look like a fifties technicolor movie. The
color killer does not make a very large impact for black and white
material (the only time it is needed), frankly I'm not sure if its not
just the placebo effect. I can live without color killer but
definitely not without chroma AGC.

> Note that a change like 'Chroma AGC must be disabled if SECAM is
> used' is a bug fix and should clearly go in.

But rather pointless as it currently cannot be enabled...

> 2) Chroma AGC and color killer is also present in other chips
> (cx2584x, cx23418, possibly other similar Conexant chips). So if we
> decide on allowing these controls I would prefer making this a
> standard control, rather than a private one.

A quick grep shows that the bttv-driver also exposes chroma AGC as a
private control. Cx2584x has chroma AGC enabled by default. Maybe the
right thing to do is to enable chroma AGC by default for PAL and NTSC?
Chroma AGC is something you'll find on most VCRs and TVs, and then it
is on by default.

> Personally I think these controls are too low-level, but that's just
> my opinion. Most chips contains a whole array of similar tweaks that
> you can do, and exposing them all is not the way to go.

Personally I'm against dumbing down the driver and not exposing
features which are useful. An argument against your stance is that the
V4L2-spec defines V4L2_CID_AUTOGAIN, V4L2_CID_AUTO_WHITE_BALANCE and
V4L2_CID_HUE_AUTO (half of the functionality of chroma AGC) which are
similar functions. Also consider the MPEG controls, most of which are
fairly obscure if you are not familiar with the MPEG specs.

Regards,

--Frej

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Re: [PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Trent Piepho]

On Sun, 16 Mar 2008, Frej Drejhammar wrote:
> Hi Hans,
>
> > 1) Should we really expose these settings to the user? I have my
> > doubts whether the average user would know what to do with this, ...

V4L2 controls aren't some menu a user must wade though when setting up
mythtv.  I don't see the harm in adding them.  There is even software for
windows that will let people mess with these things, so there is a demand.

> That was my initial take on it, therefore the first version of the
> patch just added a module parameter. I reasoned that chroma AGC was
> something you just needed to enable once depending on the quality of
> your video-source. Then Trent Piepho suggested that the functionality
> should really be exposed as controls. I think he has a point, consider
> for example living in a place such as southern Germany where you could
> receive both German PAL and French SECAM broadcasts. If you then also
> used a composite/s-video external video source you would want to be
> able to change the setting depending on your input and the channel you
> tune to.

CAGC came up before and there was a patch.  I had a patch for it, but then
Mauro changed some stuff in the driver around so my method no longer
worked.

One of the things you should do it make the control inactive when in SECAM
mode.  V4L2 has a flag to indicate controls that don't apply to the
device's current mode.

Overall, module parameters for these things is something the V4L1 bttv
driver did because controls didn't exist for V4L1.  Controls are a better
way and we shouldn't use module parameters for video decoding controls.

> > ... and I also wonder whether it makes enough of a difference in
> > picture quality.
>
> For me it does, fiddling with the saturation and hue controls I never
> managed to get neutral color reproduction. The colors were either
> washed out or saturated to look like a fifties technicolor movie. The

CAGC makes a difference for me too.  Some of my channels are over saturated
and some are under saturated and CAGC fixes them.  I don't recall if I
posted pictures last time CAGC came up, but it really does make a
difference.

> color killer does not make a very large impact for black and white
> material (the only time it is needed), frankly I'm not sure if its not
> just the placebo effect. I can live without color killer but
> definitely not without chroma AGC.

I haven't ever been able to notice an effect from color killer.  Maybe if
you had poor reception from a B&W source?  Not much black and white on
broadcast TV these days.

> > 2) Chroma AGC and color killer is also present in other chips
> > (cx2584x, cx23418, possibly other similar Conexant chips). So if we
> > decide on allowing these controls I would prefer making this a
> > standard control, rather than a private one.
>
> A quick grep shows that the bttv-driver also exposes chroma AGC as a
> private control. Cx2584x has chroma AGC enabled by default. Maybe the
> right thing to do is to enable chroma AGC by default for PAL and NTSC?
> Chroma AGC is something you'll find on most VCRs and TVs, and then it
> is on by default.

That's what I would do.  Have a standard control for CAGC and turn it on by
default.

> Personally I'm against dumbing down the driver and not exposing
> features which are useful. An argument against your stance is that the

If I wanted to be told I wasn't worthy to use my hardware, I'd run windows!

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Re: [PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Frej Drejhammar]

Trent,

> One of the things you should do it make the control inactive when in
> SECAM mode.  V4L2 has a flag to indicate controls that don't apply
> to the device's current mode.

I guess it is the V4L2_CTRL_FLAG_INACTIVE flag (in the flag field of
struct v4l2_queryctrl) you are referring to, correct?

> CAGC makes a difference for me too.  Some of my channels are over
> saturated and some are under saturated and CAGC fixes them.  I don't
> recall if I posted pictures last time CAGC came up, but it really
> does make a difference.

Good that I'm not the only one who wants/needs it :)

> I haven't ever been able to notice an effect from color killer.
> Maybe if you had poor reception from a B&W source?  Not much black
> and white on broadcast TV these days.

So maybe I should just skip the color killer then...

>> A quick grep shows that the bttv-driver also exposes chroma AGC as
>> a private control. Cx2584x has chroma AGC enabled by default. Maybe
>> the right thing to do is to enable chroma AGC by default for PAL
>> and NTSC?  Chroma AGC is something you'll find on most VCRs and
>> TVs, and then it is on by default.
>
> That's what I would do.  Have a standard control for CAGC and turn
> it on by default.

Then that's what I'll do. Expect a revised version of the patch which
enables CAGC by default for PAL+NTSC and implements the
V4L2_CTRL_FLAG_INACTIVE by the end of the week.

> If I wanted to be told I wasn't worthy to use my hardware, I'd run
> windows!

Hear, hear! :)

Regards,

--Frej

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Re: [PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Vanessa Ezekowitz]

I agree with the idea of making Chroma AGC available as a control - I've got the same problem here with at least one channel that is way over-saturated relative to the others.

That said, is this something that would show up in e.g. xawtv or other analog TV programs?

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Re: [PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Hans Verkuil]

On Sunday 16 March 2008 22:05, Frej Drejhammar wrote:
> Trent,
>
> > One of the things you should do it make the control inactive when
> > in SECAM mode.  V4L2 has a flag to indicate controls that don't
> > apply to the device's current mode.
>
> I guess it is the V4L2_CTRL_FLAG_INACTIVE flag (in the flag field of
> struct v4l2_queryctrl) you are referring to, correct?
>
> > CAGC makes a difference for me too.  Some of my channels are over
> > saturated and some are under saturated and CAGC fixes them.  I
> > don't recall if I posted pictures last time CAGC came up, but it
> > really does make a difference.
>
> Good that I'm not the only one who wants/needs it :)
>z
> > I haven't ever been able to notice an effect from color killer.
> > Maybe if you had poor reception from a B&W source?  Not much black
> > and white on broadcast TV these days.
>
> So maybe I should just skip the color killer then...
>
> >> A quick grep shows that the bttv-driver also exposes chroma AGC as
> >> a private control. Cx2584x has chroma AGC enabled by default.
> >> Maybe the right thing to do is to enable chroma AGC by default for
> >> PAL and NTSC?  Chroma AGC is something you'll find on most VCRs
> >> and TVs, and then it is on by default.
> >
> > That's what I would do.  Have a standard control for CAGC and turn
> > it on by default.
>
> Then that's what I'll do. Expect a revised version of the patch which
> enables CAGC by default for PAL+NTSC and implements the
> V4L2_CTRL_FLAG_INACTIVE by the end of the week.
>
> > If I wanted to be told I wasn't worthy to use my hardware, I'd run
> > windows!
>
> Hear, hear! :)

That's not quite what I meant. I'm responsible of all the MPEG controls, 
so I'm definitely all for exposing hardware features to the user :-)

What I want to prevent is adding controls as a workaround for what might 
be a driver bug. So in this case I wonder whether chroma AGC shouldn't 
be enabled in the cx88 driver as it is for cx2584x.

Looking at the cx25840 datasheet it basically says that it should always 
be enabled except for component input (YPrPb) or SECAM. So I would 
suggest doing the same in cx88 rather than adding a control. Only if 
there are cases where Chroma AGC harms the picture quality rather than 
improves it, then the addition of a control might become important.

Regards,

	Hans

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Re: [PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Mauro Carvalho Chehab]

On Mon, 17 Mar 2008 11:33:58 +0100
Hans Verkuil <hverkuil@xs4all.nl> wrote:

> That's not quite what I meant. I'm responsible of all the MPEG controls, 
> so I'm definitely all for exposing hardware features to the user :-)
> 
> What I want to prevent is adding controls as a workaround for what might 
> be a driver bug. So in this case I wonder whether chroma AGC shouldn't 
> be enabled in the cx88 driver as it is for cx2584x.
> 
> Looking at the cx25840 datasheet it basically says that it should always 
> be enabled except for component input (YPrPb) or SECAM. So I would 
> suggest doing the same in cx88 rather than adding a control. Only if 
> there are cases where Chroma AGC harms the picture quality rather than 
> improves it, then the addition of a control might become important.

IMO, the better would be to add both Chroma AGC and Color Killer controls as a generic control.

The default value for Chroma AGC should be changed to match the datasheet recommended
way (0 for SECAM, 1 for PAL/NTSC).

Cheers,
Mauro

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Re: [PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Frej Drejhammar]

> Hans Verkuil <hverkuil@xs4all.nl> wrote:
>
>> What I want to prevent is adding controls as a workaround for what
>> might be a driver bug. So in this case I wonder whether chroma AGC
>> shouldn't be enabled in the cx88 driver as it is for cx2584x.
>> 
>> Looking at the cx25840 datasheet it basically says that it should
>> always be enabled except for component input (YPrPb) or SECAM. So I
>> would suggest doing the same in cx88 rather than adding a
>> control. Only if there are cases where Chroma AGC harms the picture
>> quality rather than improves it, then the addition of a control
>> might become important.

The data sheet for cx2388x is not so clear. For ACGC it describes what
it does and then notes that it can be turned off. The default is off.

For the color-killer there is a similar description, and it then notes
that the color-killer can be disabled. The default is disabled.

Mauro Carvalho Chehab <mchehab@infradead.org> writes:
> IMO, the better would be to add both Chroma AGC and Color Killer
> controls as a generic control.

Is there a procedure for adding controls to the V4L2-spec? Is the
docbook source available in a public repository (not just the tarball
at v4l2spec.bytesex.org)?

> The default value for Chroma AGC should be changed to match the
> datasheet recommended way (0 for SECAM, 1 for PAL/NTSC).

I'll revise the patch to do this.

Regards,

--Frej

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Re: [PATCH 0 of 2] cx88: Enable additional cx2388x features. Version 2

[Trent Piepho]

On Sun, 16 Mar 2008, Jelle Foks wrote:

> Trent Piepho wrote:
>
> >> color killer does not make a very large impact for black and white
> >> material (the only time it is needed), frankly I'm not sure if its not
> >> just the placebo effect. I can live without color killer but
> >> definitely not without chroma AGC.
> >
> > I haven't ever been able to notice an effect from color killer.  Maybe if
> > you had poor reception from a B&W source?  Not much black and white on
> > broadcast TV these days.
>
> I would think that a chip configuration called 'color killer' will make
> the full channel bandwidth (~5MHz) available for the luma, while without
> it, the upper 1.5MHz or so is used for the chroma signal.

That's not what it does:
    If a color-burst of 25 (NTSC) or 35 (PAL/SECAM) percent or less of the
    nominal amplitude is detected for 127 consecutive scan lines, the
    color-difference signals U and V are set to 0.  When the low color
    detection is active, the reduced chrominance signal is still separated
    from the composite signal to generate the luminance portion of the
    signal.  The resulting Cr and Cb values are 128.  Output of the
    chrominance signal is re-enabled when a color-burst of 43 (NTSC) or 60
    (PAL/SECAM) percent or greater of nominal amplitude is detected for 127
    consecutive scan lines.  Low color detection and removal may be
    disabled.

I think it's when you have poor reception and can't decode the color
signal, which is in the upper part of the band.  Rather than decode garbage
color, it just turns color off.  Or maybe it's for avoiding adding spurious
color when you have a B&W signal?  I've never been able to get it to do
anything I could notice.

> When using a higher-quality B&W camera on a composite input, you will
> probably be able to see an improved sharpness in the picture with such a
> 'color killer' switched on.

Getting the full luma bandwidth would be achieved by turning off the luma
notch filter, which should be done already for s-video sources.  It might
not be a bad idea to have a control to allow this for composite signals,
like B&W surveillance cams.

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