Thursday, July 20, 2017

Are Synchronous DAC receivers reappearing?

Truth be told, of course, synchronous DAC receivers had never gone away.  While some of the earlier ones are discontinued, the DIR 9001 continues, and is often the receiver chosen by DIY'ers, I have noticed.  I'm talking SPDIF/AES only, as you may already know I despise everything else for practical/personal reasons, AND that's a pretty cut and dried case about which I need not comment anyway.

But from barely respectable on up in manufactured gear, asynchronous receivers have/had become the norm.  Few except for cranks were raising the old "puts your jitter into the data" arguments.  Some equipment was giving you a choice--that's fine.  But generally since the asynchronous receivers were showing better measured performance, anyone who cared was using them.  Except for the universe of contrarians.

I'm using one right now with my DVD-5000 dac, which has CS8414 and dual differential 1704's.  CS8414 might not have the best self-jitter, DIR 9001 may be better in that regards.

But anywayz it seems to me that synchronous receivers are required for things like HDCD, aren't they?  You can get away with any sort of interpolation with that, I would think, there's meanings to certain exact sets of bits.

Then, I also think about MQA.  Once again, it seems if you are encoding any extra information into the audio, that isn't the kind of thing which may be interpolated or whatever.

So I noticed in review of the latest and greatest Meridian DAC, which is wonderful for sure, mention is made of a FIFO buffer which gets jitter to below 0.5 Hz.

OK, that sounds like a synchronous receiver with a 1 second buffer, though I could be wrong about the buffer size.

Most of the receiver chips get away with what seem like tiny buffers, then often spec jitter suppression only above 20kHz.  That is surely wrong, it should be at least spec'd down to 1Hz.  With a long enough buffer, the DAC clock can vary slowly yet stay in line with whatever the source does, and therefore only subsonic jitter remains.  I understand that people are quite sensitive in the usually subsonic 3-10 Hz range to FM, aka jitter or wow, so you have got to get below that.

Now I really do wonder what is going on inside the Denon DVD-9000 with it's 330 msec latency compared to other DACs.  Is it an anti-jitter mechanism?  How well does that work?

I know there's no question I have to get beyond my very limited so far jitter measurements and do a better investigation of all this, with J test and so on.

BTW, the J-test harmonics appear in most cases to be way way below -110dB, often in the -130dB range.  That's a worst case jitter situation, which maybe occurs for a few seconds in a lifetime of playing.  Mostly, the jitter sidebands must be way below that.  And that's so negligible it's a wonder we even think about it.  (Well, that's a long story of course.  And it also relates to the lack of controlled blind testing.  And people don't want to write off uncontrolled impressions they have had.  And with digital transmission there can only be two things, data and time.  And the data can be checked and shown to be perfect.  So the only thing left is time.  SO time MUST explain all!!!)

This seems way below the importance of more easily measured things, like harmonic (and therefore IM) distortion, which often reaches high in audiophile designs.  Often distortion sidebands reach -60dB or higher, possibly 3,000 times or more larger than the sidebands being caused by jitter even through toslink, etc.

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