Bit perfect digital audio

In a comment to an earlier post, Fredrick mentioned that he had ‘read an interesting article by Chris Connaker at computeraudiophile.com. While testing Asus sound cards, he believes he has achieved bit perfect playback that still sounds awful’. A little while later jhans11 kindly provided a link to what appears to be the article Fredrick was talking about.

Here is the article. It is a review of a couple of Asus sound cards. The review is extremely detailed; admirably so. Any review can be agreed with or disagreed with. This review is so detailed that you can see pretty clearly why the reviewer made his judgements.

I disagree with at least one of those judgements, and indeed the reviewer himself is equivocal about the same thing: that is, his methodology for determining whether or not a sound card is passing through the digital audio ‘bit perfect’. He does this by plugging the coaxial digital audio output of the sound card into the digital audio input of his external DAC. This apparently supports the HDCD encoding enhancement. He feeds a digital signal with a HDCD flag into this, and sees whether the DAC properly detects the flag.

The flag is indicated by a particular pattern in the least significant bit (LSB) of the 16 bit signal (and, he says, in the LSB of 24 bit signals which surprises me, there being no point in HDCD for 24 bits).

HDCD is, essentially, a compander system although the company tends not to view it that way. If the flag is present, then the processor stretches the louder bits out so that they become louder, and the softer bits out as well, so they become softer. In other words, the dynamic range is extended beyond that available within the 16 bits of a CD. I hear they talk about it being equivalent to 20 bits of range.

What I don’t know is whether the HDCD flag is a continuing pattern in the digital signal (which would mean surrendering one bit of real resolution), or just a burst at a particular point at the start. With a reliable signal, the latter scheme should work fine.

Of course, with the latter scheme, kicking the HDCD system on says nothing about how accurately the subsequent bits are being conveyed, so let’s assume that the HDCD flag is ongoing.

The author thinks that because the HDCD flag is in the LSB, it is most susceptible to corruption, and therefore its working is a good indicator of signal integrity. Unfortunately this is analogue thinking. Mixing any noise (or dither) into the signal would certainly screw up the LSB HDCD flag. But in the digital world, each bit stands alone. At the risk of gross oversimplification, the receiving DAC collects 16 individual bits one after the other, and only then packages them together into the byte which is used to modulate the analogue output signal. But each of those 16 bits are as likely to be corrupt, or as unlikely, as any of the others. It is perfectly possible for the LSB to be intact, while several of the more significant bits aren’t.

The descriptions of the problems all point to digital audio data corruption. This could be timing: if the source device and the receiving device get out of sync, then either samples will be inserted or signal samples will be lost. But with S/PDIF as the carrier, this should not happen because the DAC slaves itself to a timing signal generated on the S/PDIF line by the source.

So I’d say the clicks indicate bytes corrupted by an incorrect bit (or bits). For them to be obvious, the bits would be in the more significant portion of the byte. (Corruption of less significant bits will be happening at the same rate, but less noticeably.) ¬†Incidentally, a click doesn’t require much corruption at all. Open an audio file in a digital audio editor, and move one sample by, say, 25% of the full scale either up on down. Just one sample. Play it. There will be a clear click at that point.

So what’s a good way of checking whether the output of a digital audio device is bit perfect? One way would be to use a second computer with a digital audio input to record its output. Compare the original file with the newly recorded one. If they are perfect matches you know the output of the device is bit perfect.

But that¬†presupposes that this recording computer’s sound card is bit perfect in recording. You can check this by playing a CD from the digital audio output of a CD player or DVD player into the input, recording a section. Then rip the CD and compare.

I have done this myself in the past.

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