Optical SPDIF Rx Recording has "Alias" frequency of "Fs/2-f"

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James Chan
Junior Member
Posts: 4
Joined: Wed May 11, 2022 8:46 am

Optical SPDIF Rx Recording has "Alias" frequency of "Fs/2-f"

Post by James Chan »

Hi,

My HW use XU216, I'd like to enable Optical SPDIF input and output. My software is based on "sw_usb_audio-[sw]_6.15.2" reference code.

The SPDIF Tx works well, I can record stream by another sound card.

But when I try to record Optical SPDIF Rx to a wave file, it seems there is always a "Alias" frequency of Fs/2-f.
For example, if the Optical SPDIF signal sent from another sound card is 8kHz, sampling rate is 48kHz, then the recorded wave has two frequency: 8kHz and 16kHz (=48kHz/2-8kHz).
I tested same issue in both 44.1kHz and 48kHz sampling rate.


Image

Did anyone met this issue before? Any suggestion what is the potential issue?

Thanks
James
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James Chan
Junior Member
Posts: 4
Joined: Wed May 11, 2022 8:46 am

Post by James Chan »

According to spectrum, it looking like every other sample was set to 0. but can't figure out where did it happened?
zero_samples.png
every other sample zeroed
zero_samples.png (11.13 KiB) Viewed 4809 times
every other sample zeroed
every other sample zeroed
zero_samples.png (11.13 KiB) Viewed 4809 times
The recorded wave form shows every other samples are zeros:
Recorded_Waveform.png
(31.22 KiB) Not downloaded yet
Recorded_Waveform.png
(31.22 KiB) Not downloaded yet
James Chan wrote: Sun Jun 26, 2022 5:21 pm Hi,

My HW use XU216, I'd like to enable Optical SPDIF input and output. My software is based on "sw_usb_audio-[sw]_6.15.2" reference code.

The SPDIF Tx works well, I can record stream by another sound card.

But when I try to record Optical SPDIF Rx to a wave file, it seems there is always a "Alias" frequency of Fs/2-f.
For example, if the Optical SPDIF signal sent from another sound card is 8kHz, sampling rate is 48kHz, then the recorded wave has two frequency: 8kHz and 16kHz (=48kHz/2-8kHz).
I tested same issue in both 44.1kHz and 48kHz sampling rate.


Image

Did anyone met this issue before? Any suggestion what is the potential issue?

Thanks
James
James Chan
Junior Member
Posts: 4
Joined: Wed May 11, 2022 8:46 am

Post by James Chan »

Issue solved. The SPDIF Rx channel overlapped with ADC channel.
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