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billbo 9th October 2008 09:59 AM

S/PDIF data structure
Hi guys. I was wondering on certain DVD players the S/PDIF outputs you can vary the volume level and some you cannot. Also EQ and other filters can be activated whilist using the S/PDIF. How is this done? I thought the data was a serial stream derived from the source with added control data. I can see the volume control can probably be varied by the weighting of the bits on the data but EQ and Filters has got me confused how that works. My plan is to use an old DVD player as an audio player using a new DAC I built From Hi FI (extreme TDA1543 DAC) via the optical/coaxial S/PDIF output.
My main question is does this affect the quaility of the source even though it is still digital data off the cd itself or is it churned and manipulated through those "jungle" chips which these players have in them? I am concerned that even though the signal is S/PDIF off the DSP, the signal is not the same in quality as for example the S/PDIF output of an old Sony CDP with a 2500 DSP in it.
Can anyone out there shed some light on this topic?

Billy D...

rossl 9th October 2008 11:48 AM

You are correct, the controller chip in the DVD player is manipulating the data before conversion to SPDIF.

You certainly want to set it up so no damage is done by the DVD player.

If you go into the setup page of the DVD player, you can turn off all the tone controls. There may be a setting for "raw" that will leave the data unchanged.

The volume turned all the way up will proabably let the data come through without being mangled by the cheapo DSP algoritihm.

I tested the Oppo by playing an HDCD disc and then using an HDCD decoder on the data stream. If the data were changed by the player, it would not decode the HDCD properly. You can also test this with a DTS disc and DTS decoder.

billbo 10th October 2008 12:06 AM

Thanks for your reply. The bottom line is, inside the elcheapo chips does it stay digital all the way? Or does it change it to analogue, does its stuffing around and then back to digital, Is the S/PDIF data structure equiped to handle volume, EQ and filtering inside the bit stream?
Last question. If all settings writen to off and all bypassed, Is the signal just as good as a higher end CD/DVD players? Provided I reclock the signal locally in the DAC . I would assume the jitter in those el cheapo DVD players are very bad. The reason why I like to use the el cheapo DVD players is that I like the volume control only. Not the other jargon.

Billy D...

rossl 10th October 2008 12:27 AM

No, it doesn't get converted to analog inside the DSP chip. They are manipulating the digital audio samples.

The SPDIF stream doesn't know about volume or eq, it is just samples.

You are at the mercy of whoever wrote the code that is changing the bits in the controller chip.


billbo 14th October 2008 12:16 PM

I guess I will have to find one which intervenes the signal the least.
Thanks Rossl for your input.
Billy D...

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