What's happened to CD?
17th January 2010 by Tim Jarman
Do you remember the first time you heard Compact Disc? Compared to the cheap record players and misadjusted tape decks that formed the bulk of the audio equipment in use up until the end of the 1980s it was a revelation, with its clean bright treble, absent background noise and the way that loud passages of music seemed to leap out of the loudspeakers.
One undeniable advantage that CD has over the various analogue audio formats is its dynamic range: the difference between the quietest possible sound that can be discerned above the medium’s natural background noise and the loudest sound that can be recorded without excessive distortion. CD can in theory manage 96dB, in comparison to the best cassettes at 70dB and LPs at around 50dB.
The dB (decibel) logarithmic scale is used by engineers to express the difference between two quantities that vary over a wide range. When discussing the magnitude of audio signals all that you have to remember is that the signal halves (or doubles) with every 6dB of change. The dB is a relative unit, so one needs a reference value to make any sense of it. In terms of CD players 0dB is the level at which all 16 bits in the digital code on the disc are at “1”, the highest value that can be represented and therefore the maximum output level of the DAC (digital to analogue converter) in the player. In CD terms, 0dB (strictly speaking 0dBfs, or “full scale”) is a “hard limit” that it is not possible to go beyond, in contrast the 0dB of a cassette recorder is an arbitrary signal level which long ago represented the point at which, with a certain type of tape, the amount of distortion present was 2%. With modern tapes and heads one can easily record beyond 0dB without generating 2% distortion. +4dB is not unusual when using metal tape and Bang & Olufsen’s own HX Pro recording system.
The way that distortion occurs is another difference between analogue and digital audio systems. An analogue system in theory suffers from less distortion the smaller the signal it has to handle. Factors such as noise and the non-linearity of some types of amplifier work against this simple rule, but it is still valid as a general case. Digital systems are the opposite, because the resolution of the system falls as fewer of the available bits are used to represent the signal. Thus it follows that distortion becomes worse, in fact performance improves the more the signal is expanded, right up until all the bits are at “1” when suddenly it becomes horrendous. So bad is this effect that for years CDs were recorded at a fraction of their maximum possible level, under the strict guidance of the few CD pressing organisations.
Initially the advice was that the bulk of the music should be at a level no higher than -18dBfs, that is, using only 13 of the 16 bits of digital capacity. The reason for this was to allow transients, like the crash of a cymbal or the snap of a snare drum, to be reproduced accurately and completely without distortion. The trade off was of course that the overall level of distortion present in the rest of the signal was worse than necessary, giving some early CDs their characteristic steely, edgy sound. The average level was soon revised upwards to -12dBfs, doubling the available digital resolution for the bulk of the programme content but still leaving a possible fourfold increase in signal level for the loud bits.
Under these rules, the best use was made of the strengths of the compact disc system, music was crisp and clear with brilliant, breathtaking transients which no other format could match. If you look at the specifications of an early Bang & Olufsen CD playing system like the Beosystem 5000 or 5500 you will find that even though the CD player and the cassette recorder are both line level sources the cassette recorder typically produces an output signal of around 500mV RMS at 0dB where as the CD player produces 2V RMS at 0dBfs. This is not an accident, it simply reflects the fact that most of the time the CD player should ideally be playing material that has an average level of -12dBfs, a quarter of the maximum value of 2V, which is of course 500mV. Therefore when changing between the two sources the listener would for most of the time hear no change in volume, a desirable state of affairs.
These rules prevailed for all of the time that CD was mainly a format used in large (and expensive) hi-fi systems owned and used by experienced listeners. However, as cheaper players, portables and in-car models appeared the CD went from being exclusively high-end to become a mass market product. The needs of the users of these new types of player are different to those of the serious audiophile, so the way in which CDs were made also started to change. It has long been known that people respond more favourably to music that is played loudly than to music which is soft. The non-linearity of the ear in the frequency domain plays a part in this, the listener’s perception to both the high and the low frequency extremes improves greatly with increasing sound level and a lot of what makes music interesting and satisfying can be found in these ranges. Of course any CD can be made to sound louder by simply turning the volume of the amplifier up, but there is a commercial (as opposed to a technical) advantage to be had if the discs themselves simply sound “louder” without having to do this. CDs with a compressed dynamic range can also sound clearer in car players where background noise would otherwise drown out the quieter parts of the music. The last thing a driver wants is to be startled by brief, shatteringly loud passages that suddenly rise above this level.
Remembering that the average level on a CD was set to record transient sounds accurately and that the maximum level that can be recorded on a CD is fixed at an absolute point that cannot be changed, obviously something had to give. The answer was to compress the transients and raise the average level of the rest of the music. This certainly gave a sound that was loud but the dynamic range of the signal is actually less. Unlike raising the volume control to make a quiet CD loud, there was no way that the listener at home could recover the lost information. Music without vivid transients is bland and dull, it’s difficult to put one’s finger on exactly what is wrong at first but it is instantly obvious that something is missing. Even this wasn’t enough in the quest for more volume; some producers pushed the level right up to 0dBfs and beyond. Of course with CD there is no beyond, the peaks in the music crashed into the 0dBfs barrier and could go no further so they became flat topped, loosing most of the information that had been contained within them and giving a harsh, distorted sound that could be really quite unpleasant to the serious listener.
Such practices were at their peak at around 1999 and although the majority of producers have pulled back from the brink there is still a great deal of processing that takes place when a CD is mastered in order to improve how the discs sound on cheap portables and car players. To those with high quality equipment this is a nuisance as it spoils the enjoyment of music that could be obtained if only the original standards were observed. Pop records aimed at teenagers tend to be the worst, although “digitally re-mastered” albums that appeal mostly to older listeners can be just as bad, often the earlier CD releases are preferable if you can find them. Amazingly, searching out old CDs is becoming increasingly popular among some of the hard core parts of the audiophile community. Classical music has been largely left alone by studio tricks and really benefits by CD’s huge dynamic range, a well produced disc played on one of the better Beosystems can still be an absolute joy.
The CD system is under pressure from those who profit from persuading you to regularly replace your equipment and everything you play on it with “the next big thing”. Even though one of the reasons given is the limited 16 bit resolution of what is now almost a 30 year old format, don’t believe a word of it. Try to hear CD at its best before you go the trouble and expense of buying all that music yet again.