CAS 1: Basic Theory
(1) Why use CAS?
Computer can be a “PERFECT” digital media playback machine. The word “PERFECT” means that the digital data output from computer is 100% same as what originally recorded on the media. I’m sure audiophile are skeptical about digital perfect idea. Before we go on, let’s review some basic digital audio theory.
(2) Sample rate
Define the number of samples per second. Use CD as an example, CD has 16bit/44.1kHz. which means 44,100 samples in a second (88,200 stereo samples).
(3) Digital Word-length
Digital world uses binary numbers. It includes 1 and 0 only. Wordlength means that how many digits are used to represent a number. A word-length of 4 bits (4 digits) can have values from 0000 to 1111 (in decimal, 0-15).
(4) What about CD?
CD has 16 bits wordlength, which means in every sample, it carries 16 bits of information (in decimal, the possible value range between 00000 to 65535). As mentioned on the above, CD 44.1kHz sample rate equals to 88,200 stereo samples, each with 16bits word-length. Well, it all means tons of data are represent within one second of music.
(5) A “Perfect” Ripping?
In CAS, audiophiles rip their CDs to computer harddisk and playback via harddisk. There are many programs to perform the ripping process. A perfect ripped disc means whatever data/music is stored on the CD, is identical to what is stored on the hard disk. There is NO loss of data. The same idea like you copy a digital photo from your DC to computer. They look the same, 100% the same.
(6) How can a cheap CD-Rom can perform this task?
Maybe it cannot in 52x real times, but the perfect ripping program should read the disc in multiply times and verify of all data are exactly the same. The most well known CD ripping program is called EAC (Exact Audio Copy). There is no 30kg weight Esoteric CD-Rom in this world for a reason, although I do love and respect its strong and ultra smooth mechanism.
(7) What about perfect digital audio playback?
A perfect digital audio playback means the digital audio at the output (no matter AES/EBU, SPDIF, TOSLINK or HDMI) carries the identical digital data.
(8) How can you be so sure of this?
We can test it. The procedure is simple. Connect your computer digital output to digital input and forms a loop. Record/Playback the same test signal, compare the recorded music file with the playback signal, they should be identical.
(9) I want to know a little bit more about how to compare both files? A Null test?
Import both files (File A & B) to a computer audio editor. Align both files (A & B) in sync (identical starting point) as you never can hit “PLAY” & “RECORD” buttons at the exact same time, and delay between the digital I/O.
Reverse L/R channels phase on either File A, or File B, but not both files. Playback both files together. Since the digital audio editor mixes (sum) 2 files together, they cancel each other out and there should have no signal on the playback. With serious professional digital audio workstation (DAW), this null out perfectly down to -144dBFS (24bit).
(10) What about if they are not identical?
This means there is something wrong with either your digital input or digital output. Consult your soundcard manufacturer for further investigation.
Nice work!! I think computer based audio is good as rise of internet, processing power and disk storage going really cheap. But I still like things in one box and minimal connection. and dirty power for consumer PC, and lifespan of Harddisk, I can’t bet my audio collection on PC yet, I’m I still running Vinyls, CD/SACD and some computer audio at the same time. Computer Audio still have some miles to go before becoming mainstream.
Hi, one of the biggest myth in CD Ripping is some people talk about Plextor.
Even file compare reveal the outcome WAV files are identical from a Plextor ROM and a Chick ROM,
Their comment is Plextor’s generated WAV has better sound.
What’s your comment on this?
Hello Hercules, thanks for your comment.
Hello fourcows, thanks for your question. This is non-sense. If they are data identical rip, there is no such different. If you compare their wave file, they are exactly the same.
Plextor was very famous for its quality of read and write. Even EAC recommended Plextor 8 or more years ago. On the other hand, use Plextor to write CD-R with good CD-R material can creates better sounding CD for worse DACs. This is because of the low BLER error.
These are all history now because Plextor has stopped create their own drives.
It is very good explanation for the CAS usage in plain English without under non clear professional terminology.
Keep up the good work.
Hello Kent,
thank you for useful question, I am a audiophile which want to try CAS.
i want to ask a bisic question.
I wnt to know why we can control the volume in itune ? digital volume or through the computer, I always think the message send to the DAC is only information about the music.
Will the volume control affecting the quality?
If so, can we turn off this function?
Hello nghk, to control the volume in iTunes of inside computer/digital hardware boxes, this is called “Digital Signal Processing – DSP” Since digital data are binary numbers, we can modify the numbers and change the content info. (for example volume, freq, dynamics)
Yes, the volume control can affect the quality. DSP algorithms can affects sound quality too, depends on how precise the algorithm can be made. Within iTunes, if you set the level to Max. That turn off that DSP. You should also turn off the “Sound Enhancer” / “EQ” etc.
Hello kent,
Thank you for your quick reply.
Hi Kent,
I’ve one fundemental question:
Someone said direct stream music from computer via cable is not good because of there are many noises generated by computer which is not designed for music playing, it should be connected via squeezebox ->DAC (wire or wireless).
What is your comment?
Regards
K W Chan
Hello KW Chan,
The term “noise” which audiophile mentioned should be more specific. The RFI, interface, ground noise are potential problems that may affect the quality of connection, even it is a digital one. These “noise problems” do not affect the data integrity but the timing (jitter). Jitter can be cleaned by great dejittering circuit and the best location is inside DAC. Weiss DAC2/Minerva uses firewire connection and the master clock is located inside the DAC, not taking timing information from the computer, this solves the problem too.
For squeezebox, they are just another connection method which has plus or minus. You may completely avoid the ground noise from computer, but you deal with the signal goes through (air) wireless, the locking signal accuracy is another area which may raise problem of timing.
Yo Kent, just purchased Pure Music to use with my 2010 Mac-Mini. Am using the Weiss INT 202 to interface with Anthem’s Signature D2v, which is an A/V PrePro that digitizes all inputs up to 196khz. While the sound is marginally better than Itunes (more like a tweek) I’m curious as to your opinion using Amarra. While I wouldn’t expect it to be an event, however I certainly would expect it to be noticeably better,like OH Yeah!! My equipment and ears have no trouble discerning the difference.Thanks, Peechus.