I don't have this fully formulated in my head yet. But I'm going to take a stab at explaining what I'm thinking, and I'd like to see if anyone has any detailed information to either support or refute this idea. I've been ruminating over this one for more than a month now, and it's time I got some other opinions. I know that there is at least one person on this BBS who knows this subject in excruciating detail, and I'd like to see if I can get other expert input on this.

Warning: This can be a rather deep subject, and I tend to get wordy. So this is going to be a long post. It'll take me a while to build up to my actual question- bear with me.


Definitions:

To make this discussion more cohesive, I'd like to get a few conventions out of the way.

- Digital Audio Extraction (DAE), nicknamed ripping, is, for this discussion, defined as the process of using a PC's CD-ROM drive to directly extract the 16-bit data from an audio CD and save it on the hard disk as a 16-bit PCM .WAV file.

- Encoding, or data-compressing that resulting .WAV file into an MP3 file, is a separate process, and for this discussion, is irrelevant. My question is about the actual extraction process and the resulting uncompressed bytes of audio data. MP3 data compression is not what I'm asking about and I'd like to leave it completely out of this discussion.

- Jitter Correction has two common meanings, one of which is technically incorrect. The incorrect meaning is more accurately called "Sector Synchronization", and refers to error-compensating a CD-ROM drive that can't do proper DAE. For the purposes of this discussion, I would like to assume that we're talking about a drive that can do a perfect rip without errors and does not need sector synchronization. However, the true meaning of jitter correction (the time-base correction of individual samples at playback time) might be germane to this discussion, so for this discussion let's use the correct meaning for the term.

Okay, with the definitions out of the way, on with the discussion.


The DAC Process:

When you play a CD, there has to be at least one digital-to-analog (DAC) conversion somewhere along the trip between the surface of the CD and the speaker cone which makes the sounds.

In the case of playing a .WAV file, the DAC process is done by the computer's sound card. When you ripped that .WAV file, there was no DAC involved: the CD-ROM drive read the bytes exactly as-is and turned them directly into a .WAV file without any intermediate conversion. The analog conversion only happens at playback time. Since a CD is a lossless medium (and since, for this discussion, we're assuming that no errors were induced at the ripping stage), the .WAV file should be bit-for-bit identical to the audio data that the mastering facility had when it pressed the CD.

In the case of listening to an audio CD directly, without ripping, the DAC is performed by the circuitry on the CD drive. If it's your computer's CD-ROM doing the playback, then there's a little audio-CD DAC circuit which turns the bits into analog audio and sends them down the little internal stereo cable into your sound card's mixer. Yes, the sound is coming out of your sound card, but the sound isn't being generated by the sound card, it's being generated by the DAC circuitry on the CD-ROM drive. The sound card just has a little analog mixer controlling how loud the CD's volume is.

In the case of listening to an audio CD on a consumer audio CD player, the DAC is performed by the audio CD player and sent out of the stereo line outputs to your amplifier. Same as if you listen to an audio CD on your computer (although probably using a different brand/model/quality of DAC circuit).


Jargon:

Now, when you purchase a consumer audio CD player, you often see jargon on the box stating things like "1-bit" or "20x oversampling" or some other set of terms. These refer to the method that the audio CD player's DAC circuitry uses to convert the bytes into sound. I only have a vague grasp of what these methods are, and that's the first place I'm going to need help.


How much do the DACs shape the sound?

It seems to me that the single most critical part of any digital audio playback is the DAC circuitry. This is responsible for the character of the music which comes out of the player.

Do consumer audio CD players deliberately use techniques which shape the sound a little bit? For instance, removing harshness or high-frequency noise? How do they do that? I know that the answer is "oversampling" but I'm a little fuzzy on the details, help me out here.

I also know that different CD players each do this a little differently. Some do it better than others, and those are the ones cherished by audiophiles. Note that this is not so much a question of the "quality" of the DACs, it's a question of what techniques do the DACs employ when turning the bytes into audio.


Then what about PC playback?

Well then, do PC sound card DACs use the same techniques as consumer CD player DACs to shape the sound, or do they just pump the bytes out as-is?

And for that matter, what about the Empeg's circuits? Do they behave like a consumer CD player or a PC sound card?


The real question:

If a computer's sound card uses different techniques in the DAC process than a consumer CD audio player does, then it would follow that a ripped .WAV could sound significantly different than the original CD did. The actual bytes of data are no different, just the playback circuitry.

So how much different could it sound? Just how important is this?


The implication for MP3:

So if the source .WAV file sounds different than the consumer CD player does, then how can we expect the resulting MP3 to sound as good?

We would have to make sure that the WAV or MP3 playback circuits are doing the same DAC techniques to shape the sound as the best consumer audio CD players already do. Are they? And if not, is there any processing we could do to the WAV file which would emulate what the CD player's DACs are doing?

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Tony Fabris