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sinner boy



Joined: 27/12/04
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Is there a different between SRC and ASRC?
      #1059171 - 25/07/13 11:35 AM
Hello,

I was wondering if there is a different between a sample rate converter(SRC) and an asynchronous sample rate converter (ASRC)
Is it just marking hype or are there different types of SRC?

Cheers.


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Hugh RobjohnsAdministrator
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Re: Is there a different between SRC and ASRC? new [Re: sinner boy]
      #1059180 - 25/07/13 11:56 AM
In general, anyone referring to an SRC means an asynchronous sample rate converter (ASRC) -- a device that can accept more or less any input sample rate, and provide any output sample rate that might be required. The source and output can run with completely unrelated and independent clocks.

The performance of decent modern hardware ASRCs is significantly better than any modern D-A converter, so they are, in effect, blameless as far as sound quality is concerned. Software ASRCs are not always quite so blameless, and the Infinite Wave website has a lot of interesting examples of good and bad SRC built in to a wide variety of DAWS.

So ASRCs are used at the inputs to digital mixers and interfaces, and in a growing number of D-A converters.

The alternative is a synchronous sample rate converter (SSRC), and this can only translate between integer-related sample rates, and only where the source and output are synchronised to the same clock. So it's great for up or down-sampling from 44.1 to 88.2, for example, but can't manage 44.1 to 96kHz.

So SSRCs are commonly used in plug-ins involving non-linear processing (like dynamics processing), for example, where it is advantageous to up-sample (often mistakenly called oversampling) before processing, and then down-sample the result to the host DAW's native sample rate afterwards. This makes it much easier to deal with the various distortions inherent in non-linear processes, and prevent them from causing audible aliasing artefacts.

H

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Technical Editor, Sound On Sound


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sinner boy



Joined: 27/12/04
Posts: 42
Re: Is there a different between SRC and ASRC? new [Re: Hugh Robjohns]
      #1059202 - 25/07/13 02:39 PM
Yet another fantastic explanation!
Thanks very much Hugh.


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dmills



Joined: 25/08/06
Posts: 2356
Loc: High Wycombe, UK
Re: Is there a different between SRC and ASRC? new [Re: Hugh Robjohns]
      #1059228 - 25/07/13 05:57 PM
Quote Hugh Robjohns:


The alternative is a synchronous sample rate converter (SSRC), and this can only translate between integer-related sample rates, and only where the source and output are synchronised to the same clock. So it's great for up or down-sampling from 44.1 to 88.2, for example, but can't manage 44.1 to 96kHz.




Not true!

You want to look into polyphase sinc filters.

The actual difference is that a simple resampler only has one clock that matters, if going from say 44.1K to 96K it might conceptually take a clock of (441 * 960)Khz and take an input sample every 960th clock pulse while producing an output sample every 441 clock pulses. No that anything actually does this (The logic speeds required get really silly) but you get the idea, this single clock makes the thing syncronous.

An ASRC by contrast takes an input clock and an output clock, measures the difference then resamples from one to the other on the fly. Thus the input might be nominally 44.1 (but is actually 44.0999 drifting up and down my a small fraction of a Hz with temperature), and the output might nominally be 96K, but is actually 96.02K also drifting a little with temperature, the ASRC will take the input samples and produce samples at the actual output rate (without,over a broad range caring what either rate is).

While a SRC can be either software or hardware, an ASRC is **almost** always hardware (TI make good ones).

ASRCs are pure magic when crossing clock domains, and are very useful for things like spdif inputs on digital mixers as they remove the need to sync to external work clock.

The other major use is at AD and DA converters where jitter on the clock feeding the converter chip is critical, by making this clock independent of the system word clock, and using an ASRC between the two clock domains the quality of converters reference clock can often be improved.

Note ASRC parts usually work best when the two clocks are not too close, so it is wise to pick something uncommon for the AD or DA native clock, it reduces some issues in the math.

HTH.

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!


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Hugh RobjohnsAdministrator
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Re: Is there a different between SRC and ASRC? new [Re: dmills]
      #1059251 - 25/07/13 08:26 PM
Quote dmills:

Not true!




Okay, you got me... A bit of an oversimplification. ;-)

Quote:

You want to look into polyphase sinc filters.




I did a long time ago and my head still hurts!

Hugh

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Technical Editor, Sound On Sound


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sinner boy



Joined: 27/12/04
Posts: 42
Re: Is there a different between SRC and ASRC? new [Re: Hugh Robjohns]
      #1059576 - 28/07/13 11:44 AM
wow, thats a lot to take in.
So just to really throw a spanner in the works:
Could someone explain the different between a synchronous and asynchronous signal?
I thought I knew but I'm starting to question myself!


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dmills



Joined: 25/08/06
Posts: 2356
Loc: High Wycombe, UK
Re: Is there a different between SRC and ASRC? new [Re: sinner boy]
      #1059585 - 28/07/13 12:37 PM
A synchronous signal is one synchronized to an explicit clock, a synchronous system is one which the state changes occur in response to a clock as well as the input signals.

An asynchronous system is one in which the output changes in response to input changes, but in which there is no clock.
A asynchronous signal is one which is not clocked by the clock used by the receiving system.

Note that this makes an ASRC something of a weird case, as it has effectively two synchronous halves running from different clocks with some magic in between to cross the clock domains.

If you have something like 'On the rising edge of the clock, set the output to match the input otherwise keep the output the same' you have a synchronous system (Actually a 'D' type latch).

If you have 'the output is the logical and of inputs A & B', you have an asynchronous system (No clock).

I prescribe a copy of 'Horowitz & Hill' for more detail.

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!


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Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 21588
Loc: Worcestershire
Re: Is there a different between SRC and ASRC? new [Re: sinner boy]
      #1059669 - 29/07/13 09:54 AM
Quote sinner boy:

Could someone explain the different between a synchronous and asynchronous signal?




As always, what Dan says is completely right... but I'm not sure it will have helped you much in this case!

In the context of your earlier question, a synchronous digital signal is one which has its sample rate 'locked' to the master clock of the system to which it is connected.

In a normal digital audio system, one device is selected to be the 'master' clock source, and everything else is 'slaved' to that, making them all synchronous with it. The master clock device is essentially the orchestral conductor, and everything else forms the band, all playing in time.

The aim is to make sure that everything generates, moves or receives digital audio samples at exactly the same time. This is important because, for example, if you want to mix two or more digital signals together, you have to have a sample from each of them at exactly the same time. If one sample is missing because it arrived late, you can't do the sums!

To run a synchronous system all the slave devices have to be able to be 'clocked' externally from the master clock source. This is usually either via a word clock input, or by using the embedded clocks within AES3, S/PDIF, or ADAT streams (etc), or sometimes via a video reference signal.

However, sometimes it simply isn't practical or possible to slave clock a device. For example, you might have a domestic CD player than has no external clock input, and so it will always run at its own speed, and even though that is nominally 44.1kHz, it might actually be 44101Hz, when the rest of the system is running at an accurate 44100Hz. The clocks of the two systems will drift relative to each other and sooner or later samples will get lost, resulting in clicks and splats.

Or maybe the digital source is a long way away and you can't physically get a clock cable to it.

In these cases, that source is deemed to be asynchronous -- it isn't locked to your local system clock. And that's where an ASRC comes in, because it can accept the asynchronous source, essentially extract the wanted audio data, and resynchronise that to the local clock for use within the synchronous system.

Does that make it clearer for you?

H


--------------------
Technical Editor, Sound On Sound


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