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Anti-jitter RAM buffer in DACs, Which have one ?
Pio2001
post Aug 3 2003, 10:00
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In, http://www.tc.umn.edu/~erick205/Papers/pap...html#principles we can read about RAM buffers :

QUOTE
This process is referred to as timebase correction and as stated before, any quality piece of equipment will implement it.


That has been the ritual answer to jitter concerns since I'm on the Internet.

But... if "any quality piece of equipment will implement it", then some non-quality pieces of equipment don't implement it. My question is : which ones ?

The statement "quality piece of equipment" is completely subjective and doesn't give any information. For my grandfather, a "quality" CD reader will be a portable radio-cassette-CD-speakers combo for 100 $ in the supermarket. The "non-quality" one will be the Fisher-Price toy for children from 3 to 7.
For an audiophile friend, the "quality" CD player will be the 25,000 $ Wadia CD Transport+DAC, while any player under 1,500 $ is just worthless crap.

So, do a 200 $ CD Player always have a RAM buffer ? Or is it not considered a quality piece of equipment ? What about a 300 $ CD Player from 1990 ?
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2Bdecided
post Aug 4 2003, 11:27
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Pio,

I agree with the importance of your question: like most answers which suggest that digital audio "is basically perfect in any decent equipment", people who give these answers don't really understand the truth of it.


Let's assume the incoming audio data has loads of jitter on it, or the clock can't be recovered with a decent degree of stability. Now, the ideal answer is to dump the clock and re-clock it. But there's a problem: the new clock can't be completely independent of the original one. If it's too fast, then eventually you'll be wanting to output the next sample before it's even arrived. If it's too slow, then eventually the buffer memory will overflow.

So, it's clear that, in whatever system we have, even the re-buffered and re-clocked digital audio data MUST be synchronised, in some way and at some level, with the original, jittery data.


This means that the new clock will always be a filtered version of the original one. In a simple FIFO buffer, only the highest jitter frequencies are smoothed out. In a PLL system with a small amount of memory, lower jitter frequencies can be removed. Incidentally, the greater the jitter rejection, the longer the PLL takes to lock to the incomming signal. I've used DACs that take 2-5 seconds to lock.

A system with a large memory buffer will reject all but the lowest frequency components of any jitter (say, a few Hz at most, letting nothing higher through onto the output).


I don't know how many modern DACs use these technologies. All the sound cards I've used seem to lock almost instantly, but they're not separate DACs. The old Sony DAC I had at uni locked instantly. The nice Meridian DACs we had took maybe a second or too. An SME DAC we had took about 5 seconds.

It's an easy test though: turn the DAC and CD player on - start a CD playing, and then connect the SPDIF to the DAC - it'll give you a very rough idea of the technology inside the DAC. Though I wouldn't be 100% certain of this method! There may be methods to make a system appear to lock instantly, even though the lock is poor at first.

In one box CD players, you shouldn't need to de-jitter the signal - you should be running from a clean clock to start with, and pulling the data off disc as needed.

Cheers,
David.
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Posts in this topic
- Pio2001   Anti-jitter RAM buffer in DACs   Aug 3 2003, 10:00
- - Patsoe   Since it's the cheapest thing, I can't ima...   Aug 3 2003, 19:58
- - F1Sushi   All CD players contain buffer memory between the E...   Aug 3 2003, 21:13
- - Halcyon   QUOTE As far as jitter at the DAC stage is concern...   Aug 4 2003, 09:36
- - 2Bdecided   Pio, I agree with the importance of your question...   Aug 4 2003, 11:27
- - F1Sushi   QUOTE (Pio2001 @ Aug 3 2003, 05:00 AM)So, do ...   Aug 4 2003, 12:53
- - F1Sushi   QUOTE (Halcyon @ Aug 4 2003, 04:36 AM)Does th...   Aug 4 2003, 13:44
- - Patsoe   Those are interesting side notes, F1Sushi. Could ...   Aug 4 2003, 14:05
- - F1Sushi   QUOTE (Patsoe @ Aug 4 2003, 09:05 AM)Those ar...   Aug 4 2003, 14:25
- - Patsoe   QUOTE (F1Sushi @ Aug 4 2003, 02:25 PM)http://...   Aug 4 2003, 14:44
- - Pio2001   Thanks, all of you for the answers. There was a pa...   Aug 4 2003, 14:54
- - F1Sushi   QUOTE (Patsoe @ Aug 4 2003, 09:44 AM)QUOTE As...   Aug 4 2003, 15:01
- - Pio2001   Sorry, I completely lost the link. It was an old s...   Aug 4 2003, 21:13
- - F1Sushi   QUOTE (Pio2001 @ Aug 4 2003, 04:13 PM)Sorry, ...   Aug 4 2003, 21:21
- - Pio2001   No sorry, there was no color, it was all about jit...   Aug 4 2003, 22:02
- - F1Sushi   QUOTE (Pio2001 @ Aug 4 2003, 05:02 PM)No sorr...   Aug 4 2003, 22:13
- - Halcyon   F1Sushi, thanks for the clarification! More ...   Aug 5 2003, 08:52
- - F1Sushi   QUOTE (F1Sushi @ Aug 4 2003, 04:21 PM)QUOTE (...   Aug 6 2003, 03:23
- - F1Sushi   QUOTE (Patsoe @ Aug 4 2003, 09:05 AM)Those ar...   Aug 6 2003, 04:10
- - Pio2001   QUOTE (F1Sushi @ Aug 6 2003, 05:23 AM)Could t...   Aug 6 2003, 12:49
- - TrNSZ   There is a device available from most high-end aud...   Aug 6 2003, 13:24
- - F1Sushi   QUOTE (TrNSZ @ Aug 6 2003, 08:24 AM)There is ...   Aug 6 2003, 14:17
- - F1Sushi   Found it - actually it's from Monarchy Audio, ...   Aug 6 2003, 14:36
- - Patsoe   Actually, from the description I'd suspect it ...   Aug 6 2003, 15:22
- - F1Sushi   QUOTE (Patsoe @ Aug 6 2003, 10:22 AM)Actually...   Aug 6 2003, 15:34
- - TrNSZ   To clarify on the "Monarchy Audio" DIP D...   Aug 6 2003, 15:57
- - F1Sushi   QUOTE (TrNSZ @ Aug 6 2003, 10:57 AM)To clarif...   Aug 6 2003, 16:20
- - 2Bdecided   In the current (September!) issue of Hi-Fi New...   Aug 6 2003, 16:32
- - F1Sushi   QUOTE (2Bdecided @ Aug 6 2003, 11:32 AM)The o...   Aug 6 2003, 16:42
- - Halcyon   2Bdecided, do you know of studies/measurements an...   Aug 6 2003, 19:25
- - F1Sushi   QUOTE (Halcyon @ Aug 6 2003, 02:25 PM)However...   Aug 6 2003, 19:44
- - Pio2001   You can always record an SPDIF output with an SB l...   Aug 6 2003, 21:19
- - Patsoe   QUOTE (F1Sushi @ Aug 6 2003, 03:34 PM)QUOTE (...   Aug 7 2003, 08:30
- - F1Sushi   My take on it is that the statement about "ab...   Aug 7 2003, 12:46
- - Patsoe   QUOTE (F1Sushi @ Aug 7 2003, 12:46 PM)Thought...   Aug 7 2003, 15:23
- - 2Bdecided   QUOTE (Patsoe @ Aug 7 2003, 02:23 PM)QUOTE (F...   Aug 7 2003, 15:36
- - F1Sushi   QUOTE (2Bdecided @ Aug 7 2003, 10:36 AM)QUOTE...   Aug 7 2003, 15:42
- - ChristianHJW   This thread confirms a couple of opinions i have s...   Aug 7 2003, 16:01
- - Patsoe   QUOTE (ChristianHJW @ Aug 7 2003, 04:01 PM).....   Aug 7 2003, 18:56
- - Halcyon   Shhh... don't tell them. It's reserved for...   Aug 7 2003, 20:09
- - F1Sushi   Just came across this link, which raises several t...   Aug 7 2003, 20:42
- - 2Bdecided   This thread should be in the FAQ! Cheers, Dav...   Aug 8 2003, 09:51
- - F1Sushi   Here's a Monarchy Audio DIP on a chip: http:/...   Aug 8 2003, 16:36
- - d_kay303   QUOTE (ChristianHJW @ Aug 7 2003, 07:01 AM)Th...   Aug 14 2003, 08:44
- - Patsoe   You're pointing out the exact reason why a two...   Aug 14 2003, 09:25
- - Pio2001   As 2Bdecided explained, this is only valid for che...   Aug 14 2003, 12:17


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