periods? fragments?

periods? fragments?

[David Stuart]

Greetings! Fledgeling alsa developer here,

After reading a bunch of documentation, it's unclear to me exactly what 
a period/fragment is used for. Could anyone expand on it a bit to fill 
me in?

And since I'm on the topic of documents .. I've obviously found the 
tutorials off the ALSA documentation page, and am leafing through the 
"doxygen" docs included in the devel package, but I was wondering if 
there was anything else that I should be looking at?

Dave



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Re: periods? fragments?

[Paul Davis]

>Greetings! Fledgeling alsa developer here,
>
>After reading a bunch of documentation, it's unclear to me exactly what 
>a period/fragment is used for. Could anyone expand on it a bit to fill 
>me in?

there is a hardware buffer. its N frames long. you rarely want to deal
with the entire buffer at once. its therefore divided into sections of
a certain length, often (but not always) specified in numbers of
frames. these sections are called "periods" and typically (but not
always) the hardware will interrupt the CPU every time it finishes
dealing with a "period", and you in turn get to deal with it in 1 (or
more) periods at a time. the number of periods per h/w buffer (or
alternately, their size) is a hardware parameter; how many periods
have to have been processed by the h/w before the driver will wake up
your code to read/write more data is a software parameter.

a "fragment" is an older OSS term for the same concept.


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RE: periods? fragments?

[Giuliano Pochini]

On 19-May-2003 David Stuart wrote:
> Greetings! Fledgeling alsa developer here,
>
> After reading a bunch of documentation, it's unclear to me exactly what
> a period/fragment is used for. Could anyone expand on it a bit to fill
> me in?

You need an audio buffer to send/receive data from the audio
chip. The buffer is made of two or more periods.


Bye.



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Re: periods? fragments?

[Abramo Bagnara]

Paul Davis ha scritto:
>>Greetings! Fledgeling alsa developer here,
>>
>>After reading a bunch of documentation, it's unclear to me exactly what 
>>a period/fragment is used for. Could anyone expand on it a bit to fill 
>>me in?
> 
> 
> there is a hardware buffer. its N frames long. you rarely want to deal
> with the entire buffer at once. its therefore divided into sections of
> a certain length, often (but not always) specified in numbers of
> frames. these sections are called "periods" and typically (but not
> always) the hardware will interrupt the CPU every time it finishes
> dealing with a "period", and you in turn get to deal with it in 1 (or
> more) periods at a time. the number of periods per h/w buffer (or
> alternately, their size) is a hardware parameter; how many periods
> have to have been processed by the h/w before the driver will wake up
> your code to read/write more data is a software parameter.
> 
> a "fragment" is an older OSS term for the same concept.

Caution there Paul, your definitions are wrong.

A period in ALSA is exactly the distance (measured in frames or time) 
between two consecutive periodic interrupts generated by PCM hardware.

There is no mandatory correlation between buffer size and period for 
ALSA (as there is no mandatory correlation in all hardware).

This is fundamentally different from OSS where a fragment sizes are 
forced to be buffer_size/K (with an integer K).

This is exactly the reason why I've substituted fragment concept with 
period some time ago.

-- 
Abramo Bagnara                       mailto:abramo.bagnara@libero.it

Opera Unica                          Phone: +39.546.656023
Via Emilia Interna, 140
48014 Castel Bolognese (RA) - Italy



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RE: periods? fragments?

[Jaroslav Kysela]

On Mon, 19 May 2003, Giuliano Pochini wrote:

> 
> On 19-May-2003 David Stuart wrote:
> > Greetings! Fledgeling alsa developer here,
> >
> > After reading a bunch of documentation, it's unclear to me exactly what
> > a period/fragment is used for. Could anyone expand on it a bit to fill
> > me in?
> 
> You need an audio buffer to send/receive data from the audio
> chip. The buffer is made of two or more periods.

Note really. One fragment can be used too. In that case, application must 
do timing itself (using an external timer).

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs



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