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PCM2704: PCM2704

Dmitry S
Prodigy 20 points
Part Number: PCM2704

I have some strange issue, may be somebody can decide what the reason.

I bought DIY board with PCM2704 (86M134M) and connect to laptop (Windows 7 64 bit, USB 2.0). Device was correctly recognized and driver was installed (USB\VID_08BB&PID_27C4&REV_0100&MI_00, 'USB AUDIO    DAC').

When I play music with foobar2000 using WASAPI I listen insane level of distortions. I try to play single tone 50 Hz and record DAC output. I found that DAC waveform was distorted in next way: lower half of sine period was not distorted at all, upper half has big jumps at some levels. These jumps occurs in left and right channel and repeats each period in same way.

Next, using MATLAB I generate ramp waveform that goes through all levels for 16 bit signed number (-32768..32767), play it and record.

I found next issue: for all positive values of waveform where 8-th bit was 1 waveform drops down. Negative values outputs normally.

When I modify any waveform data and zero all 8-th bit for positive values I can play signals, of course slightly distorted.

When I fit all waveform data to negative values I can play signals without audible distortions.

When I change volume using buttons on board or windows mixer, output also changes in amplitude, but with same distortions.

It looks like 8-th bit works same as 16-th bit (sign bit).

Signals with levels within (-128..127) (that didn't touch 8-th bit) plays normally.

PS: to prevent some questions I already check that problem NOT in laptop/OS/recoding/playback/sampling frequency/volume/additional sound effects/signal amplitude/.wav file parameters/usb connection/IC power/external noise/cables/soldering/voltages/smd components.

I have no SPDIF device to record PCM2704 SPDIF output.

So, my questions:

Is this IC can have such problem with one bit in data?

Where it came from: windows driver / USB transfer / DAC / IC registers?

Is it possible to understand it is original IC or bad copy.

  • 0
    TI__Guru 52683 points

    Hi Dmitry,

    Do you see this similar levels of distortion when you simply play a tone from another source? For example, what does the output look like when you play a tone from here:

    Generally, issues like this are caused by incorrectly encoded data.  When I see issues with a single bit, I usually assume there is a formatting issue.  That being said, all the formatting is done inside the device or in the standard windows driver.  

    Is your USB voltage acceptable? If you have a headphone which has too low of impedance, then you could have some supply collapse issues.  You might also ask the vendor of this hardware if they have seen this issue before.

    I cannot comment if the device is genuine.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Prodigy 20 points

    Hi Paul,

    Thanks for answer.

    Of course I check all voltages and other sources before write a post and I didn't use headphones when record signals. I check it again with Online Tone Generator - same distortions. There are no similar behaviour with other USB or internal sound cards. Also I record and decode PCM2704 SPDIF output and it outputs same data as I play.

    So, current conclusions:

    - This device still can be used as usb to SPDIF converter,

    - I can use it as 8 bit DAC with full output voltage or as 15 bit DAC with half output voltage, for DIY purposes,

    - Aliexpress is always lottery.

    And I totally agree with your last suggestion.