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Asserting Emphasis flag in S/PDIF stream
Asserting Emphasis flag in S/PDIF stream
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Old 13th January 2007, 02:10 AM   #1
vachouse is offline vachouse  Japan
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Join Date: Jan 2007
Default Asserting Emphasis flag in S/PDIF stream

Hello, there! Just joined.
Are there any chips obtainable by an amateur that can assert the Emphasis flag in S/PDIF stream?

Behringer's SRC2496 has this feature but I have to control this asserting from Mac. I may electrically "push" the SRC2496's EMPH switch from Mac. But this switch seems to be a toggle switch (I only have the manual) and Mac will not know the current status of the switch.(I may tap the LED indicator for Mac by USB but it's too complicated for me.)

My main audio source is a satellite PCM radio that transmits pre-emphasized digital audio signal with 15us/50us protocol. S/PDIF output of the PCM radio tuner is emphasized and I record it in my Mac with Apple-Lossless format. When I play recorded audio files in iTunes through USB audio interface, I have to de-emphasize them. Currently I use custom equalizer for iTunes and a simple analog RC-filter for iPod in my car. A freeware "sox" can de-emphasize audio files but it needs conversion of file format and it takes too much CPU resources. I also record analog FM radio and CD in Mac and these music files do not need de-emphasis, of course.

I'm not too satisfied with iTunes custom equalizer and planning to de-emphasize by hardware. Currently, I use Pioneer DAT's DAC connected to Burr Brown USB audio chip by S/PDIF. I may use the same simple analog RC-filter for iPod but I plan to replace my analog amplifier by full-digital amp so I need digital de-emphasis. I did simulation of the RC-filter by Spice and the result indicates phase shift in some frequency range, which is also a reason I hesitate to use a simple RC-filter.

I'm not so confident in digital audio circuit design and assembling, so the best way to de-emphasize digitally is, I think, to assert Emphasis flag in S/PDIF sound stream so that DAC or digital amp de-emphasizes the audio stream. It may be accomplished by CPLD or by very fast one-chip microcomputer, but it seems to be much easier if I can obtain a dedicated audio chip.

iTunes is so useful and I'm transferring my audio sources such as CDs, DATs, cassette tapes into HDD. So I need to control de-emphasizing from Mac.

Thanks for reading.
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Old 13th January 2007, 04:12 PM   #2
rfbrw is offline rfbrw
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Join Date: Oct 2001
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The '2496 has two emphasis LED's, one for the input section and one for the output section. If emphasis is asserted, the input section LED will light up but as SPDIF flags are considered unreliable the '2496 can assert or de-assert emphasis as needed in the output stage. I do not think the '2496 performs de-emphasis, merely display its presence or absence.

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