[Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Dirk Stockmann]

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Hi,

I have successfully compiled and installed the newest bt_sco driver and
userspace programs on SuSE 10.0 (2.6.13 Kernel). And basically it works
great: playback is fine and also the microphone is working.
The problem is that the sound coming from the microphone is overlayed
with a annoying buzz. The buzz is not very loud but clearly hearable and
quite distracting.
The base frequency of the buzz is about 266Hz. If that helps I could
post a short recorded sample.
I would be very grateful to get pointers what to do to get rid of that
buzz. I am a experienced programmer with some low-level kernel
knowledge, so I would also invest some time in debugging if I would knew
for what I should look.

Thanks,
Dirk
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Re: [Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Steven Singer]

Oooh, this is just like Jeopardy.

I'll take Bluetooth for $500.

Dirk Stockmann wrote:
> ... 266Hz ...

At what rate are SCO packets transmitted in an HV3 link?

What do I win?

> I would be very grateful to get pointers what to do to get rid of that
> buzz. I am a experienced programmer with some low-level kernel
> knowledge, so I would also invest some time in debugging if I would knew
> for what I should look.

Based on the frequency, this looks (sounds?) like an electrical
problem. Many parts of the RF will have duty cycles of 266 Hz. Even
the current demand of the module will have a profile with this
frequency (it may pulse from a few milliamps up to 50 milliamps at
this rate, with sharp edges).

I expect you're getting pickup from the electronics on the module onto
the microphone cables,or, more likely, the audio card in the PC -
they're notoriously badly shielded, you can even hear when you drag a
window round the screen.

I don't know if this information helps.

	- Steven
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Re: [Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Marcel Holtmann]

Hi Steven,

> Oooh, this is just like Jeopardy.
> 
> I'll take Bluetooth for $500.

you just gave me an idea for another column of the conference game ;)

Regards

Marcel




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Re: [Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Dirk Stockmann]

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Hi,

Steven Singer wrote:
> At what rate are SCO packets transmitted in an HV3 link?

thanks for your reply, this is already a good hint.

> Based on the frequency, this looks (sounds?) like an electrical
> problem. Many parts of the RF will have duty cycles of 266 Hz. Even
> the current demand of the module will have a profile with this
> frequency (it may pulse from a few milliamps up to 50 milliamps at
> this rate, with sharp edges).
> 
> I expect you're getting pickup from the electronics on the module onto
> the microphone cables,or, more likely, the audio card in the PC -
> they're notoriously badly shielded, you can even hear when you drag a
> window round the screen.

But when the headset is connected to my Mobile Phone there is no noise,
so it is unlikely that it is related to the headset itself. And I always
thought (maybe wrong?) that the processing inside of the PC is pure
digital. So there should be no noise on a recorded file. Besides from
that my PC has a very good audio system with no noise at all.

I think that there is a problem with the processing of the received
packets that creates the spikes in the signal.

Regards,
Dirk

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Re: [Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Steven Singer]

Dirk Stockmann wrote:
> But when the headset is connected to my Mobile Phone there is no noise,
> so it is unlikely that it is related to the headset itself. 

I agree with this diagnostic. If the headset doesn't generate noise
when attached to another device then the noise is probably not being
inserted at the headset end.

>                                                             And I always
> thought (maybe wrong?) that the processing inside of the PC is pure
> digital. So there should be no noise on a recorded file. Besides from
> that my PC has a very good audio system with no noise at all.

Hmmm, the signal from the dongle to the PC should be digital, as should
the signal from the PC to the audio card. All the processing inside the
dongle should be digital too. The only analogue stage should be from the
audio card to the speakers.

One way to confirm whether it's electrical is to record digital signal
received from the dongle into a file, shut down the SCO link and then
play the file to the PC's loudspeakers.

Are you sending a signal to the headset and, if so, are you hearing a
buzz in the headset's speaker? I'm wondering if a buzz in that
direction (coming from the sensitive microphone input on the PC) is
being looped through the air (or through the plastic) from the
headset's speaker to its microphone.

	- Steven
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Re: [Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Dirk Stockmann]

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Hi,

Steven Singer wrote:
> I agree with this diagnostic. If the headset doesn't generate noise
> when attached to another device then the noise is probably not being
> inserted at the headset end.

No noise with two different mobile phones, but with two different dongles.

> One way to confirm whether it's electrical is to record digital signal
> received from the dongle into a file, shut down the SCO link and then
> play the file to the PC's loudspeakers.

When I record "silence" from the microphone (as silent as possible) then
there is clearly the noise with about -40db power.
Is there a way to record the digital from the headset directly, before
it is processed in alsa? Similar to the way hcidump works, maybe?

> Are you sending a signal to the headset and, if so, are you hearing a
> buzz in the headset's speaker? I'm wondering if a buzz in that
> direction (coming from the sensitive microphone input on the PC) is
> being looped through the air (or through the plastic) from the
> headset's speaker to its microphone.

Sound sent to the headset from the PC (e.g. an audiobook) is without the
buzz. It is quite low quality (like on a phone) but clear.

Regards,
Dirk

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Re: [Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Steven Singer]

Dirk Stockmann wrote:
> When I record "silence" from the microphone (as silent as possible) then
> there is clearly the noise with about -40db power.
> Is there a way to record the digital from the headset directly, before
> it is processed in alsa? Similar to the way hcidump works, maybe?

In theory, if you use hcidump and extract all the SCO data, you should
just have raw samples which you can stitch together into a file.

You should be able to see the two directions independently and see
where the noise is being introduced.

I don't know if there's a speific tool, but use hcidump -w to make a
raw log and then use hcidump -x -r to read the log back later. A little
bit of perl and a pass through sox should get the data into a playable
format.

	- Steven
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Re: [Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Marcel Holtmann]

Hi Steven,

> > When I record "silence" from the microphone (as silent as possible) then
> > there is clearly the noise with about -40db power.
> > Is there a way to record the digital from the headset directly, before
> > it is processed in alsa? Similar to the way hcidump works, maybe?
> 
> In theory, if you use hcidump and extract all the SCO data, you should
> just have raw samples which you can stitch together into a file.
> 
> You should be able to see the two directions independently and see
> where the noise is being introduced.
> 
> I don't know if there's a speific tool, but use hcidump -w to make a
> raw log and then use hcidump -x -r to read the log back later. A little
> bit of perl and a pass through sox should get the data into a playable
> format.

I never fully tested it, but you can extract the SCO stream with -A in
the latest hcidump version. It was one of my fast and ugly hacks for
some security auditing of headsets at the last UnPlugFest, but we never
actually came around in using it seriously.

Regards

Marcel




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Re: [Bluez-users] Periodic noise on microphone with bt_sco (BTA-6030 Dongle, Motorola H500 Headset)

[Brad Midgley]

Hi

>>In theory, if you use hcidump and extract all the SCO data, you should
>>just have raw samples which you can stitch together into a file.
>>
>>You should be able to see the two directions independently and see
>>where the noise is being introduced.
>>
>>I don't know if there's a speific tool, but use hcidump -w

what about capturing audio using scotest in the bluez cvs (utils/test)?
That would help us find out if we are doing something wrong in our driver.

Brad


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Re: [Bluez-users] Periodic noise on microphone with bt_sco [Solved]

[Dirk Stockmann]

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Hi,

first of all I want to thank everybody for the help. I learned a lot
about bluetooth and finally got it working, so I am happy ;-).

I installed the newest hcidump and used the -A option to capture the
sound data. It already contained the noise - so it was produced by the
headset. Then I repeated the test with the mobile phone and there was
also some noise - this time.
After some test (most have looked silly) I found out that it is caused
by feedback between the RF, the speaker and the microphone. It
disappeared when I held the headset to the ear with my hand - that must
have been the case with the first test with my mobile.
I applied a small strip of self-adhesive aluminiumfoil to a specific
part of the headset, the noise is now completely gone.

Dirk

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