Re: ADC clock drift
- From: Marcel Müller <news.5.maazl@xxxxxxxxxxxxxxx>
- Date: Sun, 25 Jun 2006 13:59:48 +0200
Hi!
mmagliaro schrieb:
For years, my company has used the simple Soundblaster Live! Value in
videoconferencing stations that we manufacture and sell. It is simple,
cheap, and unglamorous. But the sampling clock is dead-on accurate at
8000, 16000, 22050, 32000, 44100. From Windows, you can set the sampling
rate (at least on playback) to virtually any amount, and that card locks
on and plays accurately at that rate (i.e. 16008, 7992, etc).
Err, no offence, but the EMU10k chip of the SBLive operates always at 48kHz, whatever you tell windows. The resampling is done in the DSP and obviously the chip did not miscalculate the sampling rate.
That card is no longer made. The new 24 bit card is a big crap-out in
this regard. It is accurate at 8000, 16000, 32000, and 44100 (within 0.1%
or better). But at 22050, it is horrible. It actually captures at 22175,
125 Hz too high! Even worse, when simultaneously capturing and playing
on the card, if I tweak the playback rate by a few Hz (i.e. say I was
playing at 22050, and I change it to 22042), IT AFFECTS THE CAPTURE RATE!
This is a common feature of (as far as I know) all cards without a locked sampling rate of 48kHz. Otherwise there were no fixed phase relation between input and output when operating in full duplex mode. This would make the card entirely unusable for this purpose.
I don't know about using a single oscillator, or how they used to do it on
the old "Live!" card, but that was a simple cheap card, and its clocks were
dead-accurate. So this can't be that hard.
Use a $10 soundcard and your frequencies will be as accurate as with the SBLive. They all operate at 48kHz. Of course, the resampler may be not of that quality level, but you can do this in software just as well.
There is really no excuse for
a card that can't keep time, for God's sake.
Well, the higher the multiplier of the PLL, the more critical is the jitter. And noone uses 22,05kHz. It's mainly for compatibility with old games and bad sounding system sounds.
You should know what you are doing when you use consumer components for measurement purposes.
Laptops are notorious for this. I've tested a bunch of them here, and
they notoriously are hundreds of Hz off on both the playback or record
side.
Well, on the box is written "with 16 bit sound".
There is nothing said about tolerances or if at least the half of the 16 bits contain useful information.
Even worse, many of them won't actually change their rate when you tweak
it by a few Hz. The one in my Compaq Evo notebook does this. Set it on
16000, it plays at 15780. Set it to 15990, it plays at 15780. In fact,
you can step down in little steps, and it won't budge, until you get to
about 140 Hz, and then it takes a MASSIVE leap downward by about 300 Hz to
16720! It's like their driver only supports large integral steps in
frequency.
It's a question of resampling againg. Switch to 48kHz and do the resampling on your own and you will get any sampling rate you like.
Marcel
.
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