video DAC

Discussion in 'Blu-ray & DVD Players & Recorders' started by dexter, Jun 16, 2004.

  1. dexter

    dexter
    Standard Member

    Joined:
    Jan 27, 2002
    Messages:
    214
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Ratings:
    +0
    Looking at a variety of players I am confused about the quality of video DACs. Many of the better players seem to have 12/108 DACs and the more budget ones have 12/54 DACs. The Cambridge Audio Azur 540 has the "lower spec" (?) DACs (12/54) but has SIX (!) of them. Does this mean that it can compete with the higher end models that have 12/108 DACs?

    Thanks

    Dexter
     
  2. DTS0077

    DTS0077
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    Good question.....I always wanted to that aswell! Come on guruZ......provide some answers! :clap:
     
  3. dexter

    dexter
    Standard Member

    Joined:
    Jan 27, 2002
    Messages:
    214
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Ratings:
    +0
    someone must have a suggestion!
     
  4. John Dawson

    John Dawson
    Member

    Joined:
    Jan 16, 2001
    Messages:
    852
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    43
    Location:
    Cambridge, UK
    Ratings:
    +112
    I lecture on this stuff to the trade from time to time and I have to say there is a lot of marketing-ese (aka bullsh-t) deployed in video specifications in the same way that it is nowadays de riguer to have 24 bit audio DACs in audio equipment, when they typically don't deliver more than about 17 bits of dynamic range and sometimes a lot less. I could go on (but won't).

    DVD is an 8 bit video medium. In order to accomodate the sync pulses, colour bursts etc that also make up an analogue video signal derived from DVD it is easy to show that you need a "perfect" 9 bit video DAC to accomodate all this. In practice a well engineered 10 bit DAC will therefore do a more than satisfactory job. In terms of sampling frequency you need a minimum of 13.5 MHz for interlaced video and 27 MHz for progressive video. So for most purposes 54 MHz is sufficient, allowing gentler slopes on reconstruction filters etc.

    Having said that all the digital processing that goes on before the DAC inside the MPEG decoder needs to be at a significantly higher precision, say 12 bits or more. A lot will depend on how well this is done, redithered etc and it is nigh on impossible to find such information in the public domain, or even under NDA. However it is certainly true to say that MPEG decoders (the heart of all consumer DVD players) vary quite a lot in terms of absolute performance.

    I can tell you that the excellent Zoran Vaddis 5 MPEG decoder (the BGA version not the less expensive and less well specified 5E one) that Arcam uses in the DV78/88Plus/89 family has 6 x 10 bit 54 MHz DACs integrated into it and produces excellent video quality. I happen to know it uses 12 bit internal processing too. In further support of this argument the highly regarded and very expensive TAG DVD players also used the internal 10 bit DACs integrated into the Mediamatics MPEG decoder and were always praised for their excellent (interlaced only) video quality.

    We use an external 12 bit 208 MHz sampling video DAC/encoder on the DV79, with suitably readjusted filtering and the picture is arguably better than the DV78 but IMO the differences are not huge by any means. Like all such external DACs it is coupled back to the MPEG decoder via an 8 bit digital video bus.

    Of course the video quality also depends on having good properly buffered analogue reconstruction filters after the DACs and a set of proper 75 ohm buffers afterwards, plus noise free power supplies etc. IMO inexpensive and mid range DVD players rarely pay as much attention to these as is really required.

    Incidentally the minimum number of video DACs you need on a DVD player is 4 to provide RGB plus sync on composite video (i.e. to a SCART equipped display). Having 6 allows you to support s-video simultaneously (all Arcam players do this with interlaced video) but it is not essential in most set ups.

    In general you get what you pay for, but judging a player on the spec of its video DACs is like judging a car on the size of its engine - it affects the result but there are many other factors that determine how well the car actually performs and drives. There is no substitute for having a proper demonstration at a good knowledgeable dealer of either a car or a DVD player.

    And we haven't even mentioned sound quality yet :)

    HTH.

    John Dawson (Arcam)
     
  5. dexter

    dexter
    Standard Member

    Joined:
    Jan 27, 2002
    Messages:
    214
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Ratings:
    +0
    A great and helpful response - thank you. Looks like I need to save up for an Arcam!

    Dexter
     
  6. Denner

    Denner
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    You'r the man John Dawson :smashin:
     
  7. Jeff

    Jeff
    Well-known Member

    Joined:
    Jul 13, 2000
    Messages:
    5,507
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    106
    Location:
    Basingstoke
    Ratings:
    +263
    No DAC is even better. :)
     

Share This Page

Loading...
  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice