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Why not 24-bit CD?

Reader Peter Griffin asks why CDs have to be recorded in 16-bit format

On the theme of vinyl vs. CD, the article in Jazz Journal January 2011 unfortunately left one question hanging in the air: why does a CD have to be in 16-bit format? My understanding is that each "bit" is recorded along the CD track one after the other. In which case, why can't 20 or 24 of them be assembled and decoded to achieve the greater depth of sound?

The article states that when an original sound source is processed, it is often converted to 24-bit format, before final output to 16-bit format on the CD. But where is that 24-bit format stored during the conversion process? Surely on a hard disc, which in terms of technology is scarcely different from a CD, apart from its vastly greater capacity.

My suspicion is that technology has little to do with the issue. Presumably recording on a CD in 24-bit format would occupy 50 percent more space than in 16-bit format. Thus the capacity of the CD would be reduced from 80 minutes to about 50, rather similar to an LP, thus making the CD far less market-friendly.

This overlooks the huge improvement in sound quality which a 24-bit CD would offer, and also the great reduction in storage space which any CD has over LPs. Perhaps it's time to call back your "expert" and put some further questions to him. There may indeed be reasons why a CD genuinely cannot be recorded in more than 16-bit format, but they were regrettably not presented in JJ's last article on the subject.

21 April 2011

Your Comments:

Posted by James, 17 August 2011, 12:50 (1 of 2)

In case you are interested, 24 bit audio is 3 times the data of 16 bit audio in stereo. 1 minute of 16 bit is is around 10mb vs 33mb for 16 bit audio. A CD would also struggle to play 4.39 mbit/sec as it can only sustain 1.35mbit/sec. Hope you find this interesting, James. (Broadcast Audio Engineer)

Posted by Darrel Sheinman, 21 August 2011, 10:47 (2 of 2)

I don't have time to give a deep, detailed technical answer, but suffice it to say, one can Google and find out all the answers.

Briefly, the simple fact is multi-track recorders vary between 16 bit and 44.1KHz sampling rate and 24 bit with 96KHz sampling rate.

The bits refer to the amount of data captures, and the sampling rate is how often the music is read per second. These together give you the size of your data file i.e. bit rate, measured in mbit/second.

Without going into the maths (which is not meant to sound arrogant, as even I don't follow it!), 24/96 basically triples the bit rate, which is why most CDs are recorded at 16/44.1 in order to get the data on one disc. Pretty much all burners burn at 44.1 KHz too.

Only the SACD kit is better, and if the recording has been burnt at a higher bit rate, then many argue that they reproduce close to analogue, as higher bit rate allows for more "space" between the data by enhancing noise floor and headroom. It is noise floor and headroom that makes analogue sound better, by the way.

Vinyl fans, check out Gearbox Records

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