CD Audio datarate? 150KB/sec for a 172KB/sec audiostream?! :-0

[Leo V]

This is a topic I've long ignored. It has now become crucial, since I'll be backing up my music CD collection in digitally-extracted (meaning .WAV) form. Here goes:

Digital CD-quality audio takes up:
(2 channels) * (44100 samples/sec) * (16 bits per sample) / (8 bits in a byte) / (1024 bytes in a kilobyte) = 172.3 KB/sec (this is fact). To the best of my knowledge, .WAV files on your computer take 172.3KB for each second of audio.

However, 1X-speed audio CD's invariably play at 150KB/sec (this is presumably also fact). Furthermore, CD music MUST be stored at 150KB/sec in order to fit exactly 74 minutes of audio into 650MB of space.

Since CD music is of the above format, how in h3ll does it store this information? All I could find online was this info:

"A 44.1kHz rate means there are 44,100 chops every second, each one describing the waveform amplitude at that moment in time with a 16-bit number; 16-bit itself offering 65,536 steps from which to choose. With samples occupying two bytes on each of two channels - each sample yields a data transfer rate of just over 176KBps. A single-speed CD-ROM transfers data at the same rate, but a portion of the data stream is taken by error-correcting information - reducing the effective transfer rate to 150KBps. A CD can hold up to 74 minutes of encoded stereo audio data - which, when the ECC overhead is taken account of, equates to the standard CD capacity of 680MB."
Source
(They used 1000bytes in a kilobyte instead of 1024).

This only confuses me more: does CD audio omit sound information when truncating the datarate from 172 to 150KB/s? Most importantly: if WAV files take up more than raw CD audio tracks, one may be unable to fit my albums on 650MB CD's!

Does anyone understand how this business works? Please deposit your thoughts below.
--leoV

 

[Mark R]

A CD actually stores approximately 740 MB of data - which if you work it out comes to 74 x 60 x 172.3 kB

Since the data rate for CD audio is 172 kB/s - a 1x audio CD must play at 172 kB/s by definition.

However, when you record data only 650 MB of space is available, the other 90 MB is used for additional error detection and correction codes - Raw CD data has a significant error rate, while this is not a problem for audio, it would be devastating for data, hence a much stronger error correction system is used.

After the error correction has been factored in, the data rate is 150 kB/s when reading/writing data.

Basically, what this means is that the laser reads/writes 172 kB/s to/from the disc irrespective of the CD format. If the disc is an audio disc, data is transferred to/from the computer directly. The computer must be capable of transferring data at 172 kB/s when recording audio (at 1x).

If the disc is a data disc, the additional 22 kB/s are automatically calculated or discarded by the drive, meaning the computer only sees a data rate of 150 kB/s.

 

[Leo V]

Thanks a lot.

If I understand you correctly:

1) Music CD's have dismal data integrity, which allows for sufficient music storage (and makes good ripping so difficult)

2) Therefore, storing albums in .WAV form on a data disc may exhaust the 650MB or 700MB of available storage, albeit the integrity is near-perfect.

Is that right? I appreciate your help
--Leo V

 

[Mark R]

Exactly.

There is third option, which I didn't mention for simplicity, and that is Mode2/XA data.

This is an intermediate form, there is considerably less error correction than the normal 'Mode 1' format, although still slightly more than pure audio.

This format is used for video CDs, or some game data discs because it gives about 700 MB of usable data capacity on a 74 minute disc. Its use should be restricted to non-critical data such as multimedia streams.

 

[Eug]

<< Music CD's have dismal data integrity, which allows for sufficient music storage (and makes good ripping so difficult) >>



Somebody understands. Just to change the subject a bit... Whenever I tell people that good ripping is not an easy thing, they think I'm an idiot. Even consistent ripping is not the same thing as good ripping. I have a drive that consistently starts a song ever so slightly off on the disc no matter how many times I try it, but the files are always exactly the same. Most of the time I don't notice it, but every once in a while I get an extraneous sound for a split second at the beginning of a ripped song. My other drive is also just as consistent, but more importantly it starts at exactly the right spot every single time.



<< Most importantly: if WAV files take up more than raw CD audio tracks, one may be unable to fit my albums on 650MB CD's! >>



Yep. Take a 74 minute CD. Rip it and save the songs to .wav files. Burn the .wav files back as data onto a standard CD-R. Oops... you can't because there is not enough room on the disc. However, this is almost never an issue, because people don't burn .wav copies of CDs. Either they'll burn an audio CD (with the same lame error correction) or else will convert it to MP3. If you REALLY wanted to save the .wav files then the easy way out is to just spend the extra 50 cents for an 80 minute CD-R.

 

[Leo V]

I really appreciate your help. Since I'm archiving for reliable backup, I'll stick with high error correction on 700MB media (Ricoh Platinum). I must say though, it's pretty sad that music ships with such low integrity. Consumers ought to demand any less-than-perfect digital CD's to be replaced...
--leoV

PS: with the industry attempting to encrypt multimedia content, such little errors would render an entire digital track completely unusable. In any case, I hope that never goes through...

 

[Leo V]

Eug, that's exactly what I did: I bought a 50-pack of Ricoh's 700MB/80min USA-made Platinum media (they claim 200-year longevity). I also bought a 25-pack of double slimline jewel cases, to eliminate the chances of CD surfaces being scratched. (For use with my newly-acquired Plextor 12/10/32 ). My goal is reliable CD-based data storage (including post-extracted music), and I'm certainly not compromising on error correction here

 

[obeseotron]

If you've ever used nero to burn an audio CD it will state something like 62min 740MB at the top. The above explanations are correct, and until dvd-audio becomes standard (don't see why it would be at this point, but in the future) high error protection won't be built in. Another example is an MPEG1 movie that won't fit on a 650MB CDR, but if it is burned as a VCD instead of an MPEG1 file, it will only take 60 something minutes of the CD