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Difference between PCM1796 and PCM1792A

Other Parts Discussed in Thread: PCM1792A, PCM1796, PCM1792, TPA5050

Hello,

My name is daniel and I am new to this forum. I am interested in Hifi and I have some questions regarding to Audio Converters of Texas Instrument.
I own a Soundcard with an PCM1792A Chip. The sound is quite good but like all Hifi freaks i always try to increase my sound quality. So i was looking for a Digatal Analog Converter (DAC). I have found an intersting product from Atoll (DAC 100) which uses an PCM1796. Now my question. What is the differnce between the to chips? I read the specifacation of the two Chips but I dont understand much. For any kind of Information i would be very thankful.

best regards, Daniel

  • Hi Daniel,

    from what I've seen from the datasheets, the PCM1792A has got almost double the output current of the PCM1796 (7.8mA vs. 4.0mA). This helps to get the Signal-Noise-Ratio down.

    Regarding distortion, the PCM1792A is spec'ed a bit better at low fs, at fs=192kHz they're equal. In the performance charts (THD vs. Supply, Fig.11) it even looks like the PCM1796 is better at fs=192kHz.

    Another difference is the digital filter. The attenuation in the stop-band is higher for the PCM1792A.

    Also the delay is spec'ed different (slow/sharp rolloff): PCM1796: 18/fs resp. 55/fs ; PCM1792A 38/fs resp. 38/fs. In case you use both DACs in one system at the same time this might be an issue.

    I think there must be a bug in the datasheet for the PCM1796. It is not reasonable (at least at first glance), that there is no difference in delay for slow and sharp rolloff.

    --> Can someone from TI comment on this?

    Mathias

  • Gents,

    The PCM1796 is the little brother to the PCM1792.

    The PCM1792 is a larger device, that has a larger digital filter in it, providing better stop band attenuation. It also has more current segments.

    Many thanks

     

    Dafydd Roche

     

     

  • Hi Dafyd,

    thank you for confirming my observations regarding "current" and "stopband attenuation".

    Can you be so kind to comment on the delays stated in the datasheet regarding different rolloff characteristics?

    I.e: Is my understanding right, that there's something wrong with the numbers for the PCM1792A?

    I understand the delay is different compared to the PCM1796 since a different digital filter is used in the PCM1792A.

    The reason why I'm asking is, that a high-end soundcard uses the PCM1792A and the daughterboard (additional channels for surround) uses the PCM1796. This introduces a delay between these channels.

    Many thanks

    Mathias

  • Mathias, 

    let me double check with the design team on this.

    However, a different digital filter would not only have a different stop band attenuation etc but also a different latency (or "group delay").

    In a multichannel mixed converter system, the best way to compensate for this would be to buffer the "faster" DAC's up to the "slowest". This could easily be done in the software driver, or in the DSP/FPGA feeding it. (heck, you could even use a TPA5050).

    This is a common issue in recording studio's that may be using soundcards/audio interfaces from different manufacturers, and can actually be compensated for in most DAW software.

    Cheers

    Dafydd

     

     

  • Hi Dafydd Roch,

    did you get an answer from the design team yet?

    Dafydd Roche said:
    let me double check with the design team on this.

    Thanks and Best Regards

    Mathias

     

  • Dafydd Roche said:

    The PCM1792 is a larger device, that has a larger digital filter in it, providing better stop band attenuation. It also has more current segments.

    How much less current segments in the PCM1796?


    Sincerely,

    Maxim

  • Maxim,

    the maximum Current output of the PCM1792 is 7.8mA, the PCM1796 ~ 4mA.

    /Dafydd

  • Dafydd Roche said:

    the maximum Current output of the PCM1792 is 7.8mA, the PCM1796 ~ 4mA.

    Dafydd,

    So in the PCM1796 Advanced segments DAC digital input bus divided differently than in the PCM1792?
    not Upper 6 bit - MSB + Lower 18 bit, both in the PCM1792, but for example, 5 upper - MSB + 19 Lower?


    Maxim

  • Hello Dafydd,

    did you get an answer from the design team regarding the delay of the digital filter (see above)?

    Best Regards

    Mathias