Since its introduction in the 1960's, the popularity of the Philips Compact
Cassette format has resulted in it becoming the most used method for audio recordings
made on portable tape equipment and has supplanted the use of reel-to-reel recorders in
home audio systems. Broadcasters and others have abandoned using heavy professional quarter inch equipment in favor of smaller, lighter, more compact cassette systems, but this move has not been without its problems. Here we will discuss the archival transfer of important cassette materials.
Cassettes use a ribbon of tape only one eighth of an inch wide, and a linear speed of 1.875 inches per second. Both the size of the tape, and the running speed, are half that of the previously popular consumer quarter-inch tape and equipment. The original monaural cassette machines used a track width that roughly equated to the quarter track format of the quarter inch consumer equipment.
When the stereo cassette was introduced, the track width was cut to less than half of the monaural track. This resulted in decreased signal to noise figures and made track alignment much more critical. Considerable advances in tape manufacturing technology have compensated for many of the deficiencies, resulting in much lower drop out rates and much higher performance. This has attracted most people to this convenient format, whose performance now exceeds its quarter inch predecessor.
Two major cassette problems were the uniformity of alignment, of the cassette tape path, and head azimuth from machine to machine. To a large degree, the problem of tape path alignment was solved, fairly early in the evolution of cassette tape equipment, by a pair of fixed guides installed on the record/play head during manufacture. This caused the tape to follow a more-or-less fixed path across the surface of the head, keeping the tracks within a reasonable position on the tape being recorded, and ensuring that a tape being played back would also follow the same path.
The problem of head azimuth remains, and although it is largely corrected by more precise manufacturing techniques, and alignment of the head azimuth at the factory, the earlier problems still plague the archivist and anyone trying to transfer original recordings made on the older cassette mechanisms.
This problem will likely never be noticed if the recordings are played back on the machine that made them originally, since cassette recorders, with a few notable (and expensive) exceptions, use the same head for playback as they do for recording. In some cases, there are optimised playback and record heads within the same head casing, but these also suffer the problem of the single record/playback head... mis-alignment will not likely be noticed when a tape is played on the same machine that did the recording.
Results of mis-alignment of this nature, when played on other cassette machines, vary from high frequencies that "weave" in and out, to a generally muffled loss of most high frequency content. In the case of speech recordings, all the fricative sounds (the "K", "F", "S" and such sounds) are lost or severely attenuated, which drastically reduces the intelligibility of speech. A musical recording sounds predominantly bassy with all the treble range missing. Once transfers of this nature have been made, the resulting poor recordings can never be fully compensated for, and return to the original recordings is mandatory to produce an acceptable result.
The fault can easily be corrected by an attentive recording technician and involves nothing more than carefully adjusting the playback head azimuth in the machine playing the original cassette. The adjustment is simplicity in itself, but it must be performed on each cassette before attempting to transfer it.
The playback equipment must have an easy access to the azimuth adjustment screw, while the machine is running, and without the necessity to disassemble any of the mechanism. An older Pioneer CT-F8282 Stereo Cassette mechanism is one example that allows outside access to the azimuth adjustment, while the cassette itself is latched into position within the cavity by three steel pins that precisely fix the cassette position.
The actual azimuth adjustment can easily be performed while playing the cassette to be transferred. Combine the output of both stereo channels to mono, and adjust for maximum high frequency response and clarity of the material being played back. No pun intended, but the correct position will be clearly obvious, since the sound will be muffled on either side of the correct position. Rocking the adjustment screw slightly about the point of best high frequencies, and clearest sound, will assure you of the actual correct position. Occassionally, an oscilloscope will be a useful assistant.
All of the foregoing presumes that the machine being used for playback of the original cassette is operating precisely at a speed of 1.875 inches per second, and that the mechanics of the transport, and its electronics, are within the manufacturer specifications. The tape heads and any metal guides must be demagnetized with a head demagnetizer designed for cassette machines. The heads themselves should be in like new condition with no sign of wear-grooving that could mis-align the tape to be played, and lastly, the heads, tape path, rubber pressure roller and capstan shaft must be carefully cleaned and in good condition.
If the speed of the original recording equipment was not within specifications, adjustment will be necessary, and can be done as a part of subsequent restoration processing. Particularly in the case of music recordings, it is important to ensure that the restored material will play back at the correct speed and that the musicians are heard to be playing in the correct key and the correct pitch! A digital quartz metronome such as the Seiko SQ-88 or a common "pitch pipe" will produce a tone that will assist in setting correct speed/pitch for accurate playback.
Pay particular attention to the take up and hold back tensions. Mis-adjustment here could irreparably damage a tape, that might not be replaceable, or introduce excessive wow or flutter in an otherwise good transfer. The "record safe" tabs should be removed to prevent the possibility of accidentally putting the machine in "record" mode and erasing the cassette you are attempting to play.
With all of the above details attended to, you are now ready to transfer the
best possible quality from the original cassette.