10 Jan
2007
10 Jan
'07
5:43 a.m.
Quoting reuben firmin <reuben.firmin(a)gmail.com>om>:
With adequate buffering I think this shouldn't be
a problem in my use
case
-- the speakers connected to the wireless device will be in a different
room. You're not saying that a soundcard will get so far behind that the
buffers will fill up, are you? Both soundcards are designed to output in
~"real time" after all.
Buffering doesn't fix the problem at all. It will maybe postpone it, but
nothing more.
Being "out of sync" translates to the fact the soundcards will consume
samples at different speeds. While the other one will consume 44101 samples
per second, the other one might consume 44030 samples per second.
Because of this, the sound emitting from the soundcards will drift apart.
Also, the pitch of the sound will differ.
Example: Just after 10 seconds, the 1st card in my example will be emitting
sample 441010 (44101 * 10), and the other card will be emitting sample
440300 (44030 * 10). This means that if the original audio has been recorded
at exactly 44100, the cards are already 0.016 seconds apart from eachother!
(44030 is an example of a really crappy clock chip on a card, but it's not
unheard of)
Let me re-iterate: Without syncing the soundcards, this /will/ happen. This
is not a legend or some far-fetched possibility that people speak of but
which really never affects real life.
Just imagine two wall-clocks. Can you really expect them to keep the exact
same time over years of continuous running without anyone re-setting them?
Sampo
10 Jan
10 Jan
11:38 a.m.
New subject: [linux-audio-user] Outputting same sound source to multiple cards simultaneously
On 1/10/07, Sampo Savolainen <v2(a)iki.fi> wrote:
Example: Just after 10 seconds, the 1st card in my example will be
emitting
sample 441010 (44101 * 10), and the other card will be emitting sample
440300 (44030 * 10). This means that if the original audio has been
recorded
at exactly 44100, the cards are already 0.016 seconds apart from
eachother!
(44030 is an example of a really crappy clock chip on a card, but it's not
unheard of)
OK, so what is the workaround? Couldn't the card that is behind be made to
catch up between songs? Couldn't every 1000th sample be dropped on the
crappier card? Etc. In any case, in your example, the cards would be about 1
second out of sync for every 20 minutes of play time. In my use case, this
really does not matter, as, again, the speakers are in different rooms. If
the software can be made to cope (which it surely should be made to, if the
problem is as prevalent as you describe) then afaic there is no problem.
Let me re-iterate: Without syncing the soundcards, this /will/ happen. This
is not a legend or some far-fetched possibility that
people speak of but
which really never affects real life.
I touched on this above. So how do I sync the cards? Is there a specific
method to do this with alsa?
Just imagine two wall-clocks. Can you really expect them to keep the exact
same time over years of continuous running without
anyone re-setting them?
If they were hardware being run by the same software on the same PC then,
yes, I would expect them to (be made to) stay in sync, regardless of their
mechanical quirks.
Cheers
Reuben
4:04 p.m.
New subject: [linux-audio-user] Outputting same sound source to multiple cards simultaneously
On Wed, 2007-01-10 at 08:38 -0800, reuben firmin wrote:
OK, so what is the workaround? Couldn't the card
that is behind be
made to catch up between songs? Couldn't every 1000th sample be
dropped on the crappier card? Etc. In any case, in your example, the
cards would be about 1 second out of sync for every 20 minutes of play
time. In my use case, this really does not matter, as, again, the
speakers are in different rooms. If the software can be made to cope
(which it surely should be made to, if the problem is as prevalent as
you describe) then afaic there is no problem.
There are programs which work around this issue by resampling the stream
to the other sound card on the fly. These systems can work pretty well
and can be a true solution when you can live with the loss of quality.
I'm pretty sure there are applications which attempt to match sound
cards which are not synched, but i wouldn't bet on finding an
application which does /exactly/ what you are looking for.
I touched on this above. So how do I sync the cards?
Is there a
specific method to do this with alsa?
To sync cards, both cards need a common clock source. This is done in
hardware. Professional soundcards use a protocol called "wordclock"
which uses a terminated BNC cable between the cards to transmit the
clock impulses. This can't be done in software.
If they were hardware being run by the same software
on the same PC
then, yes, I would expect them to (be made to) stay in sync,
regardless of their mechanical quirks.
Don't forget that your computer is 50% hardware. Not everything is
controlled by software. The clock impulses I'm talking about are the
clock impulses which trigger the AD / DA converters. The equivalent in a
wallclock is the actual clock crystal which drive the mechanism.
Sampo
12 Jan
12 Jan
5:43 a.m.
New subject: [linux-audio-user] Outputting same sound source to multiple cardssimultaneously
Since the output from the two cards do not actually need to be in sync (the
output is in different rooms and I take it these are not recording rooms)
then can you not just feed the two cards from separate streams and just
resync/restart the stream either at silence or between songs by draining the
buffer? That way you just feed each device at the rate at which it wants to
accept data.
If you really wanted to do this from a single open file descriptor then you
could monitor the available buffer space on each device and have one device
attempt to track the buffer occupancy of the other. I would advise
resampling over sample insertion/removal because even at very low rate
differences the action would be repetitive and sample drop/insertion then
becomes very noticable. Resampling would introduce a little but of
wow/flutter into one of the streams as its tries to maintain the same rate
as the monitored stream, but hey, that can sound good at low rates - it
would be like a set of tape heads. The reason you cannot directly implement
such a solution is that every soundcard believes it is running at exactly
44100Hz (or 48kHz or whatever you configure), and they cannot tell you what
their true running rate is.
To be honest, the other option would be to just dump a block of samples if
you had overruns on a device (or repeat a block if you had underruns which
is perhaps easier to detect). If you had, say, 64KB of buffer space then the
proposed delta from your calculation below gives you about 70 excess samples
per seconds, so you would have a sound similar to a 'tick' on a CD every 20
minutes, something that is iritating at worst.
Anyway, there is no 'canned code' that will do this for you, so you might
have to get your compiler out and write the program yourself, so good luck,
audio device programming is a pain in the butt.
Regards,
Nick.
From: "reuben firmin"
<reuben.firmin(a)gmail.com>
Reply-To: A list for linux audio users
<linux-audio-user(a)music.columbia.edu>
To: "Sampo Savolainen" <v2(a)iki.fi>
CC: A list for linux audio users <linux-audio-user(a)music.columbia.edu>
Subject: Re: [linux-audio-user] Outputting same sound source to multiple
cardssimultaneously
Date: Wed, 10 Jan 2007 08:38:15 -0800
On 1/10/07, Sampo Savolainen <v2(a)iki.fi> wrote:
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Example: Just after 10 seconds, the 1st card in my example will be
emitting
sample 441010 (44101 * 10), and the other card will be emitting sample
440300 (44030 * 10). This means that if the original audio has been
recorded
at exactly 44100, the cards are already 0.016 seconds apart from
eachother!
(44030 is an example of a really crappy clock chip on a card, but it's not
unheard of)
OK, so what is the workaround? Couldn't the card that is behind be made to
catch up between songs? Couldn't every 1000th sample be dropped on the
crappier card? Etc. In any case, in your example, the cards would be about
1
second out of sync for every 20 minutes of play time. In my use case, this
really does not matter, as, again, the speakers are in different rooms. If
the software can be made to cope (which it surely should be made to, if the
problem is as prevalent as you describe) then afaic there is no problem.
Let me re-iterate: Without syncing the soundcards, this /will/ happen. This
is not a legend or some far-fetched possibility
that people speak of but
which really never affects real life.
I touched on this above. So how do I sync the cards? Is there a specific
method to do this with alsa?
Just imagine two wall-clocks. Can you really expect them to keep the exact
same time over years of continuous running without
anyone re-setting them?
If they were hardware being run by the same software on the same PC then,
yes, I would expect them to (be made to) stay in sync, regardless of their
mechanical quirks.
Cheers
Reuben 
10:33 a.m.
New subject: [linux-audio-user] Outputting same sound source to multiple cards simultaneously
On Wed, 2007-01-10 at 08:38 -0800, reuben firmin wrote:
On 1/10/07, Sampo Savolainen <v2(a)iki.fi> wrote:
Just imagine two wall-clocks. Can you really
expect them to
keep the exact
same time over years of continuous running without anyone
re-setting them?
If they were hardware being run by the same software on the same PC
then, yes, I would expect them to (be made to) stay in sync,
regardless of their mechanical quirks.
but the audio interfaces are not "being run by the same software".
control over how they see time belongs to the clocks on each interface.
the PC cannot control the clocks in anyway. this is why if you want
things to stay in sync, you must share a clock.
note: one of the very first tasks in designing any kind of digital audio
installation that involves multiple devices (of any kind) is to
determine what the clock sharing network will look like. getting it
right is critical. the fact that these cards share slots on the same bus
doesn't remove the issue.
--p
6628
days inactive
6630
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linux-audio-user@lists.linuxaudio.org
4 comments
4 participants
participants (4)
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Nick Copeland -
Paul Davis -
reuben firmin -
Sampo Savolainen