Using Audio CDRs
...a special report compiled by SRTL to clarify the mysteries relating to recording audio CDs. This item was written some years ago, and is, in part, somewhat out-of-date.
CD-R Audio discs are not the same as as CD-R data discs that play audio. It is important to know this - more will be revealed in the course of this article.
CD-R continues to be the most popular medium for a variety of audio applications including pre-mastering, back-up, archiving, reference copies, DJ previews and pre-published music distribution. Ten years ago a recordable CD cost 15 UK pounds (24 dollars), today you can pay as little as 20 pence (30 U.S. cents). Cost and quality issues mean cassette duplication is now all but dead and the reliability factor means broadcasters view CD-R as preferable to DAT for transmission purposes. Although DVD in its various recordable formats gained increasing prominence in those areas where greatly increased capacity is required (film, video, surround sound etc.); CD-R remains the low-cost, high-quality medium for audio work. To be certain that every disc duplicated will sound and play as we expect it to, we need to understand more about this medium we've come to rely on so heavily.
The Problem With CD-R
As the musical instrument industry's foremost independent distributor of CD-Recordable media and hardware, SRTL has a good deal of insight into the pitfalls of audio recording on CD-R media. Many of today's musical recordings are made on second-rate or inappropriately chosen CD-R media. In most such cases, problems only manifest once the disc has reached the listener. Audio quality does vary as we shall discuss later, but the overwhelming problem caused by inferior or inappropriate media is lack of playback compatibility. When we choose our blank media without regard to performance criteria, we can unwittingly 'infect' our recordings by generating huge error counts in the encoded data. Whilst we may have saved a few pence on our blank CD-R's, or simply picked up the brand that our local dealer stocks, we can never be quite sure how the recordings will perform on CD players 'at large'. In most cases, the anomalies are erratic - track 5 jumps on this disc, track 7 won't play at all on another and some discs simply won't register their presence in a player. Why? - because there are huge differences in the ability of CD players to correct these unwittingly recorded errors. What's more, you can't check compatibility unless you've got a) expensive test equipment or, b) a roomful of different CD players and a lot of spare time.
Choosing The Right Brand Of CD-R
Many sound recordists now wisely choose audio-optimised media for everything but in-house listening or other non-critical copies. They've learned of the pitfalls the hard (and expensive), way. In response to the particular requirements of audio recordists, a small number of manufacturers are now marketing and specifying such audio-optimised CD-R media. Of these, most however remain OEM (that is, sourced from a limited menu of specifications available from an un-named manufacturer), although highly respected brands such as TDK do specify and produce the CDR's themselves to their own extremely exacting standards. The reputations of such companies cannot be built on the back of advertising campaigns; rather by producing media that records flawlessly, is highly durable, offers maximum compatibility and that has come to be trusted by those in the know. A note of caution here: because counterfeit CDR media is being passed-off or unwittingly offered as name brands by some suppliers, you should always check that what you're buying is European specification, legitimate product. A good supplier will only offer you factory fresh media from legal sources - if in doubt, or if the price looks too good to be true; then leave well alone.
Why Sound Varies On Different Types Of CD-R...
How sound is recorded
When we record a CD-R, the laser power output is increased in order to heat the transparent polycarbonate and dye layers in the discs structure. As the heated dye layer collapses forming a 'pit', so this transition obscures the reflective layer (the mirror-like silver or gold 'surface') and denotes a change in status of the binary digit - from '1' to '0' or vice versa. Jockeying back and forth like this the sound waveform is represented as a 16 bit string of 1's and 0's. However, if these 'pits' in the melted dyes are not formed accurately then the 16 bit string will produce a different waveform when decoded in the DAC, that is to say, a different sound is heard. There are other issues which can affect sound such as the length between transitions and the actual shape of the formed (mis-formed), data pits themselves. For brevity though, we can appreciate from this simple description that it's not just 'a load of 1's and 0's' etched into some shiny disc.
Do different coloured discs sound different?
Opinions vary on what dyes and combinations of reflective layers produce the 'best' sound. This is of course a subjective issue and whilst colorations in sound may be evident between differing brands, it's fair to say that only very poor 'B grade' un-named CD-R media are likely to cause offence to the ears. [N.B. Don't confuse un-named with un-branded which simply means the named manufacturer is producing a surface free from trademarks]. So yes, there may be slight differences in the sound of one brand or specification over another; but it should be remembered that the real issue, the most likely problem area, is going to be playback compatibility rather than sound. No need to get hung-up on specifying a gold reflective layer over a silver one, a green rather than a blue dye, pthalocyanine to plain 'ol organic cyanine/azo formulations when looking for a 'good' sounding CD-R. In terms of sound at least, these are by and large obsolete distinctions rendered irrelevant by technological developments and to get caught up here is to be sold into the hyperbole of the differing manufacturers' marketing departments.
So what are the differences then?
For the record, a gold surface is sometimes claimed to last 200 years rather than 100 for silver; though no-one actually knows and of course we won't be playing CD's in the 23rd. century anyway. Also of little import to most of us is that if you're planning on leaving your CD-R's in direct sunlight for long (why would you?), then pthalocyanine dyes resist UV rays better. Much more importantly for all of us, is the accepted fact that a silver reflective surface (regardless of dye colour), is more widely compatible. This is the crucial issue because if others can't actually play your discs it doesn't matter how good they sound on your machine.
About CD-R Write Speeds...
Do write speeds affect sound?
It is now widely accepted that audio CD-R recordings should be made at the slowest practicable speeds - ideally in real-time. Reported effects of high-speed (say, 6x or higher), recordings in apparent sound are loss of fullness in the bottom-end and a meddling of the stereo image. Some very prestigious studios claim astonishing differences in subjective sound when recording at these higher speeds and will insist on real-time recordings when mastering or for highest-quality listening copies. At the same time, the investigations engineers at the Production Resources Department of the BBC are quoted as saying they've found no appreciable sound difference when recording between 1x and 4x (but no faster). Even though opinions may vary slightly when it comes to quantifying the effects of record speed on sound, it remains a crucial issue when it comes to our most important consideration - that of playability.
Do write speeds affect playability?
There is now an established consensus that write speed does affect the playability (compatibility) of CD-R media. Even though our own recorders may be capable of correcting high error counts on CD-R's burnt at high speed and therefore will not manifest any playback problems on the recording machine itself, we can assume that problems will obtain on (at least some), other players. To counter this previously widespread problem, a few manufacturers are now producing audio-optimised CD-R media which is designed to reduce BLERs (block error rates) to an absolute minimum. Additionally, dyes can be modified to offer far greater stability than standard cyanine dyes on recordings made at 1x to 4x (500-2,000 rpm.). An example of this would be TDK's CD-R "XG Audio", now known as CD-R Audio which utilises a proprietary (specifically modified), cyanine-based dye in the recording layer. Utilising media such as this and limiting write speeds on duplication work to no more than 4x, one can be sure of the highest possible degree of compatibility. But remember, our more critical reference copies and mastering work should always be recorded in real-time or at most 2x using the same, audio-optimised media (i.e. TDK's "XG Audio").
About 80 Minute CD-R
The 80min/700Mb CDR is now commonly available though is not part of the 'Orange Book' industry standard. Accordingly, we need to be aware that problems may (no longer an issue - SRTL )arise dependant on what hardware we are using and the quality of the disc itself. Abbey Road reported early problems with cheap CDR-80's producing very high error rates and the only discs that came up to standard on their 'Stagetech' analyser were TDK and Philips Professional (PDO). The production tolerances for 80min/700Mb CDR media are extremely low and only the latest and most sophisticated production facilities are suitable. To get an idea of these tolerances we can look at some specifications:
The laser of a CD player follows a track on the disc surface, the distance between each track being less than 2/1,000ths. of a millimetre. Reducing the gap by 1/10,000th. of a millimetre allows an additional 50Mb or 6 minutes of audio to be added. If we need the extra capacity of an 80min/700Mb CDR we must only use only the highest quality product. Even so, some older recorders and software cannot recognise the new standard and will only record to 74min/650Mb or reject the disc entirely. Drives showing problems include the Sony CRX100, HP8100 and Philips CDD3610 - all fine recorders in their own right but not compliant with the changing standards in media. Consumer-type CD recorders are not affected at all by the use of 80min/700Mb CDR media (i.e. TDK CD-R80 "XG Audio").
About Re-Writable CD-R...
How CD-RW works.
Unlike write-once CD-R, the re-writable medium uses phase-change technology in a mixed metal alloy recording layer rather than melting pits into dyes. The phase of this alloy layer changes with the power of the recording laser and thereby its' reflectivity is altered - as with the melted pits in CD-R technology, this transition signals a change in the binary digit status. The reflective layer would be an aluminium alloy rather than the silver/gold of its' CD-R counterpart. Due to these technical differences, the output (write power), of the recording laser needs to be increased from 6-7mW to 8-14mW and most audio CD players are unable to read data from the CD-RW discs' surface due to their low output (read power).
Do we need CD-RW for audio?
Re-writable (CD-RW) media could be viewed as a precursor to the emergent re-writable DVD media or as an evolution from (and potential successor to), write-once CD-R. In audio terms though, we could see the format more as a potential replacement for DAT as a mastering medium; at least amongst semi-professional users and the wider home recording community. Although a good quality CD-RW disc can be written, erased and re-written in excess of 1,000 times, there is no evidence yet that such extensive usage impacts negatively on quality or reliability as would certainly be so with digital tape. Now this may be due to the fact that the medium itself has not been widely adopted in pro-audio circles, and reports on performance are therefore limited. Given the conditioning amongst professionals that only virgin media should be used, there is reluctance to take advantage of the apparent durability and flexibility of the medium. The fact that CD-RW media is now only about twice the price of CD-R (its' disposable counterpart), is unlikely to sway the not so cost-conscious professional user into adopting the format more widely. Accordingly, usage is most extensive amongst consumer/semi-pro users who acknowledge the distinct benefits of being able to use their CD recorders as mastering devices as well as duplicators and who appreciate the economic advantage of re-usability. Further to this, the professional environment is more security conscious when it comes to protecting master recordings and not being able to erase data on a (write-once) CD-R is regarded as advantageous given that eraseability of stereo recordings can be affected in workstations, on DAT etc. Finally, the lack of multi-read functionality in most CD audio players means CD-RW media is rightly perceived as being of low compatibility in audio terms at least. For archiving purposes it should be noted that rewritable media has a lower life expectancy than write once CD-R and in any event it would rarely make sense to use the more expensive medium for this purpose. If generally accurate, this combination of factors means CD-RW will struggle to become the natural successor to CD-R in the audio industry.
About Consumer CD-R...
Why do we need another type of CD-R?
Desktop CD recorders such as those produced by Philips, Pioneer, Akai and some Marantz units, require the use of a bit-encoded, or 'Consumer' CD-R. In an agreement made in Athens in 1992 between the manufacturers of these low-cost, 'Consumer' recorders and world-wide copyright authorities, this new disc specification was defined in order that differing territories could impose levies on the blank media to compensate for the (assumed), loss of revenue from illegally-copied material. Of course, this would penalise anyone using these low-cost recorders for the purposes of duplicating their own or non-copyright material. However, only selected territories choose to impose the levy and it is not imposed in the United Kingdom. When you buy this type of media in the U.K. you should be aware that it is sold to you 'ex-levy' and copyright must not be infringed.
How does a 'Consumer' CD-R differ?
A 'Consumer' CD-R has it's type defined by the statues of bit 14 in the lead-in track - a 'Consumer' CD recorder will check for status during the initialisation process (this is when the laser output is adjusted to optimum power for the formulation of the particular disc in use). If the recorder cannot read the necessary bit-encoding then its' display will simply read 'no disc' (or some other such message), and no recording can take place. The best 'Consumer' CD-R media will be audio-optimised so as to produce maximum compatibility on a wide variety of playback devices (see section on CD-R write speeds elsewhere). As a 'Consumer' CD recorder will record no faster than 4x (2,000 rpm.), deleterious results may well be experienced using media optimised for 8x to 48x (the norm for standard data CD-Rs), even though such media will of necessity include the bit-encoding necessary to define it as 'Consumer' type. That is to say, some brands of 'Consumer' CD-R will simply be a data disc (8x to 48x), with the bit-encoding added - no account is made of the intended record speed. As always, the problem is compatibility which is the absolutely crucial issue for recording musicians distributing their work for others to listen to on a wide variety of players.
A Final Word About CD-R For Audio.
We hope the information we've presented here is of interest and will help you make informed media buying decisions in the future. Apologies must be conveyed to anyone who feels we're teaching them how to 'suck eggs' - our knowledge comes in part from our customers feedback and we're indebted to all who are kind enough to relay their experiences to us.
It should also be said how ironic it is that so many people spend so much time and money producing their multi-track recordings only to duplicate their stereo mixes onto inferior CD-R media. After all, these CD-R's do look the same don't they? And surely one binary digit sounds the same as another doesn't it? Well, now we know this just isn't true.
You may well have spotted that here at SRTL we trumpet the cause of TDK optical media products in particular. The reason for this is that we believe TDK consistently offer the best combination of value, performance and reliability. Alternative recommendations include Philips Professional (PDO), whose award-winning DVD and CD technology also meets the stringent standards demanded in high-level audio recording. It is difficult for us to make such broad recommendations on optical media from brands which, whilst the name on the disc may be well-known, are in fact out-sourced media products (invariably from one of the vast, Far-Eastern production facilities), and in such cases feedback from users suggests specifications can vary on a batch-to-batch basis. This may be attributable to refinements and/or variations in raw materials which would not necessarily be identified in the branding companys product codes. We are sorry at having to add that last, extra variable to the plethora of caveats the audio recordist should rightly concern him or herself with in order to assure success; and we do so not to cause added confusion, but merely for the sake of a rounded perspective.
In closing, we wish you well with your creative endeavours and recordings and trust that this article has contributed to your knowledge of audio recording with CD-R.