Revolutions in sound recording
Revolutions in sound recording

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Revolutions in sound recording

3.4 Compact cassettes

The use of magnetic tape for home use has always been somewhat problematic. Whilst it offers several advantages over discs, being capable of high-quality sound, substantially free from surface noise and able to make personal recordings, tape never became so popular as to make any serious inroads into the sales of discs. Why should this be the case? The answer is one of convenience, for magnetic tape has always been difficult to handle compared with discs – threading the tape through the machine and onto the take-up spool was a fiddly process, and the tape could easily get damaged or snap.

Many companies developed tape cassette systems based on standard quarter-inch tape but none succeeded in gaining acceptance by consumers. The compact cassette system, shown in Figure 24, was developed by Philips Gloeilampenfabrieken in 1963 for recording speech (shades of Edison!). Philips called their cassettes compact to distinguish their system from other audio cassette systems and they made no pretence of achieving high-quality sound, deciding to use a slow tape speed (1 7/8 ips) and a new narrow one-eighth-inch-wide tape to keep the whole system as small as possible. The convenience of slotting cassettes into the machine rather than having to thread tape around guides and tape heads made this format much more suitable for consumers.

Figure 24: A Philips audio cassette recorder with a compact cassette

To use the compact cassette system in place of vinyl LPs necessitated overcoming two obstacles: first, the limited bandwidth due to the low speed, and second, the poor signal-to-noise ratio because of the low signal level output from the narrow tape. The bandwidth was increased to a degree by the use of special magnetic tape formulations, including high-density ferric oxide, chromium dioxide and pure metal compounds. The signal-to-noise ratio was also improved because these tapes allowed signals to be recorded at higher levels. However, the poor signal-to-noise ratio was really only solved by the Dolby Laboratories, who developed and licensed a consumer version of their professional noise-reduction system, Dolby A. The Dolby B noise-reduction system described in Box 8 dramatically improved the sound quality on compact cassette tapes, enabling them to rival discs. Remember that although Dolby B encoding can reduce tape hiss, it cannot be used to improve the quality of the original recorded sound.

The ability to make Dolby-encoded home recordings was a very attractive feature of the system and certainly contributed to the wide acceptance of the compact cassette. This was exploited particularly in automobile audio systems where a copy of an LP or CD could be played whilst driving. Sales of classical music compact cassettes overtook LPs by 1983 but were themselves overtaken by CDs in 1988. By 1994 classical CDs took 78% of the market, compact cassettes 21% and LPs a mere 1% (data from the Statistics Handbook (1995), The British Phonographic Industry, London, p. 21).

Box 8: The Dolby B noise-reduction system

Dolby Laboratories developed their Dolby B noise-reduction system to improve both the frequency response and the signal-to-noise ratio of the compact cassette system.

Magnetic tape can hold only so much signal; beyond this it will saturate (i.e. the magnetic particles on the tape cannot be magnetised any more). The louder the signal being recorded, the closer the tape becomes to being saturated. For a particular tape recorder and tape the amount of tape hiss is constant. Thus the louder the wanted sound, the less obtrusive the hiss will be. However, if loud (high-level) signals are recorded and at the same time boosted significantly for the purpose of noise reduction, the tape would saturate and the recording would become distorted.

The basis of the Dolby noise-reduction system is that low-level high-frequency signals are boosted when the recording is made, and the opposite process is carried out on replay. The process is applied only to high-frequency signals, as this is the frequency range where the hiss is most obtrusive.

To boost the noise-reduction effect further, the Dolby B system uses a sliding range for the frequency where the signal boost starts to happen. When the sound signal is very low or contains few upper frequencies, the boost start point slides to its lowest frequency, giving on replay a maximum of 10 dB noise reduction above 4 kHz. As the sound level increases and/or there are more higher frequencies in the signal, the start point frequency rises and so the perceived reduction in noise on replay is reduced; but of course because the signal is louder in the upper frequencies, the hiss is less noticeable.

The key to successful operation of this system is in the positioning of the sliding bands, and the ability of the decoder in the replay machine to track these changes in frequency and so reproduce exactly the original signal. If, for some reason, the frequency response of the encoded signal is changed before it reaches the decoder, mis-tracking of the sliding bands will occur. How audible this becomes has to do with several factors, including the nature of the music, the listening conditions and the sensitivity of the listener. However, audible effects of mis-tracking are minimised by limiting the overall boost range to 10 dB.

Activity 22

Disc recordings have always had a specific advantage over tape when it comes to accessing a particular part of a recording. By describing the different technologies used to store the sound, can you suggest what that advantage might be?

Answer

On the tape the sound is recorded as a series of magnetic fluctuations along its length. In order to get to a particular part of the recording the tape must be wound forwards or backwards. This may take several minutes, especially if the required sound is at the other end of the tape.

On discs the sound is also recorded serially as a single spiral track. However, to find the equivalent sound on a disc is a relatively quick operation, performed by simply placing the pickup at the appropriate place on the disc surface. This takes the same time no matter where it is on the disc. The speed and ease of access to particular songs has always given an advantage to the disc over tape as a commercial replay medium.

Activity 23

Listen to the audio track below. You will hear an original live digital recording made at The Open University especially for this activity. During the recording four different formats were used: direct digital, an original compact cassette tape without noise reduction, the same tape with Dolby B noise reduction and a metal-compound cassette tape again with noise reduction. Listen carefully to the differences in the quality of the recorded sound.

Note: the differences are quite subtle and you may find them easier to distinguish using headphones (but remember to check your volume levels first if you are switching to headphones).

Click below (1 minute 38 seconds)

Download this audio clip.Audio player: ta212_3_006s.mp3
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Answer

Comment

I expect you noticed the very intrusive tape hiss after about 18 seconds. I am sure you agree this is unacceptable for most music recordings although it could be tolerated for speech. The tape hiss is reduced to an acceptable listening level after a further 25 seconds by using a Dolby B noise-reduction processor. This cuts the tape hiss by about 10 dB but maintains the tonal balance of the sound. This would not be possible using conventional filters (e.g. treble-cut tone controls), which although they would suppress the tape hiss would also affect the tonal balance of the sound. Finally, after a further 30 seconds the tape hiss becomes almost inaudible through the use of a metal compound cassette tape and Dolby B noise reduction. This combination cuts the hiss by a further 10 dB, making the overall sound quality very close to the original digital recording at the beginning of the track.

TA212_3

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