cd_read_side.jpg (8K)

Chip's CD Media Resource Center:
Article: Is It Better To Record At Slower Speeds?

Image from Disctronics

Is it Better to Record at Slower Speeds?

I alluded, in the Introduction, to my project of recording CD audio interviews with family members. Just before I finished somebody asked me if I was recording at 1x, and said that they had heard that 1x recording was better. Well, I was recording at 8x and hadn't noticed any problems, so I blew them off.

But as I later read through Andy McFadden's CD-R FAQ I came to item 3-31, Is it Better to Record at Slower Speeds? which explains that the answer is more complicated than a simple "yes" or "no". Here is item 3-31:

It depends on your recorder, media, and who you talk to. For example, some informal testing with the venerable Yamaha CDR-100 determined that it worked best at 4x speed with media certified for 4x writes. 1x worked almost as well, but 2x would occasionally produce discs with unrecoverable errors.

With audio CDs, the results are more subjective. Some people have asserted that you should always write at 1x, others have stated that 2x may actually be better. It depends on the recorder, media, player, and your ears. Try it both ways and listen. See section (4-18) for some notes on how you can write the same set of bits to two CDs and still have audible differences.

CD-R media is written by heating up tiny sections of the disc. When the disc spins faster, the laser has less time to shine on a particular spot, so the laser has to be controlled differently. Different formulations of media may require a different "write strategy" at certain speeds, and each recorder may adjust its write strategy differently to accommodate those speeds. This can potentially result in combinations of recorder and media that work perfectly at one speed but fail miserably at another.

Put simply, there's more to writing at high speed than just spinning faster. There is no One True Answer to this question. Do what works best for you.

See "The Speed of Sound: How Safe is High-Speed CD-Audio Recording?" at http://www.emedialive.com/EM2000/starrett5.html, for a very thorough analysis of audio disc quality at several different speeds. With some recorders and some media, it's actually better to write faster -- but in none of the tests performed did the error rate get anywhere near danger levels, regardless of speed.

See the graphs at http://www.digido.com/meadows.html for an examination of BLER (BLock Error Rate) with different recorders, different media, and different recording speeds. A few of the graphs show the same recorder and same media at different speeds, and in some cases the BLER increased at higher speeds, while in others it decreased.

See http://www.cd-info.com/CDIC/History/Commentary/Parker/stcroix.html for commentary about "write strategy" selection and different media types.

In the article "The Speed of Sound: How Safe is High-Speed CD-Audio Recording?" at http://www.emedialive.com/EM2000/starrett5.html, Robert Starrett notes that, "some people question whether advanced recording speeds might have an adverse effect on the underlying data, the longevity of the disc, and its playability across multiple brands of CD-ROM and DVD-ROM drives and consumer audio and DVD players." These are the questions he seeks to answer.

As I see it, there are thus three parts to this question:

  1. Data integrity
  2. Data longevity
  3. Interoperability

Data Integrity

Does recording speed affect the number and type of errors in the data? Starrett observes that "All discs have errors on them. Luckily, most of these errors are corrected in playback. Physical error rates like Block Error Rate (BLER) can measure the quality of a disc. The final quality assurance in an audio disc is, of course, the listener's own ear."

I was amused by his comment about "the listener's own ear", since I'd be aiming for a disc with zero uncorrectable errors. But I suppose he has point -- the error correction codes used on audio-format discs simply don't go as far as those on data discs. Audio playback hardware will, if necessary, interpolate adjacent samples or blank the lost sample in order to mask (not "correct") an error. So an error rate that leads to uncorrectable errors could indeed affect playback. In case I needed further humbling, there are some additional factors mentioned in the CD-R FAQ, section 4-18-2, that can explain why a CD-R may sound "different."

Starrett also claims that the error rate will affect longevity and interoperability, but surprisingly presents no rationale or evidence why this should be so. I think his thesis is that on a disc with a high error rate, it will only take "one more fingerprint" or small scratch to push it beyond acceptability. That has some merit, but isn't quite what I was looking for.

However his experimental procedure and results for testing data integrity are the best I've seen so far, so any other flaws in his article are forgiven.

Here's what he has to say about BLER:

BLER stands for Block Error Rate. It indicates the number of data blocks that have any bad symbols (bytes) at the C1 [primary] error-correction stage. The reason that BLER is a good indicator of overall disc quality is that it essentially indicates the number of all errors, since errors that are uncorrectable at C1 get passed to C2 [the secondary error correction stage]. The Red Book specifies a maximum BLER of 220 per second, averaged over 10 seconds. Top-quality discs have an average BLER of below 10. A peak of 100 bad-data blocks per second is acceptable for CD-ROM, but an average BLER of 50 per second over the entire disc is a good cut-off point to ensure data integrity. CD-Audio discs can be acceptable with high BLER, but high BLER indicates that their longevity may be limited, and that there may be some problems in playing reliably on all types of readers.

Read the article for yourself and you'll see that CD-Rs recorded at any speed actually have much lower BLERs than stamped audio CDs. So record as fast as you wish. I like the way he summarizes:

Let the timid confine themselves to recording at a snail's pace with the hope of improved audio quality. You know better.

Regrettably, Starrett's article does not clearly specify the details of the disc blanks that he used. He tells us the manufacturers, but not the reflective layer or dye types. The implicit message is that "it doesn't matter".

Data Longevity

Does recording speed affect the longevity of a CD-R? Starrett's approach was to say, in effect, that "it will if it makes the error rate too high." But it didn't, so "case dismissed." I find myself wondering if the high recording speed could possibly affect the stability of the pits burned into the CD-R. No data on this so far...

Interoperability

Does recording speed affect in any way the ability to read a CD-R on various kinds of equipment? Having already ruled out error rates as an issue, I speculate that interoperability might be affected by tracking and eccentricity problems. No data on this so far...


Last Updated Monday October 15, 2001 17:58:04 PDT