FOM2 is a recording of spot frequencies for checking the frequency response of record playing equipment. It starts with 20sec of 1kHz (announced as 'One thousand cycles per second') and is followed by 15k, 12k, 10k, 8k, 6k, 4k, 2k, 1k, 500, 200, 100, 60 and 40Hz for approximately 10sec each and ends with 10sec of 1kHz again. The recorded level is about 21dB below the peak level of programme found on a typical LP.
As can be seen from the label it complies with the British Standard Fine Groove Characteristic BS 1928:1955 with time constants of 3180, 318, and 75µs specifying turnover frequencies in round figures of 2120, 500 and 50Hz. This curve is identical to the RIAA curve and is the playback curve that has been used for all commercial 45 and 33 1/3-rpm records since 1953.
Prior to 1953 the situation was somewhat chaotic with record companies using their own ideas on equalisation for fine groove recordings in much the same way that they did for 78-rpm records. (In the early 50s it used to be said that a good record should sound like hi-fi on equipment that isn't!) Since standardisation seemed impossible, in 1950 the Audio Engineering Society of the USA proposed a standard playback curve hoping that manufacturers of record players and amplifiers would adopt it. It was thought that this would then force the record manufacturers to produce records that would sound OK when played on this equipment. The curve was designed so that even if record manufacturers did not accept it, the errors on playback, given the variety of equalisation in use at the time, would not exceed 2dB. It would also give acceptable results for most contemporary 78-rpm discs provided that they had been recorded with some high frequency pre-emphasis.
In 1953 the curve was slightly modified and adopted by all concerned. It was known initially as the New RIAA - AES - NARTB - RCA New Orthophonic Playback Curve and later adopted as the British Standard. The curve is still used unchanged today except for the IEC amendment introducing an additional time constant of 7950µs for a turnover frequency of 20Hz which is intended to reduce rumble, but many amplifier manufacturers apparently ignore this on the assumption that their customers will be using high quality turntables.
SOM11 is an oddity dating back to 1951. Not long after the AES curve was proposed, the CCIR in Geneva, June 1951, proposed a compromise characteristic for radio programmes for international exchange. This had time constants of 450 and 50µs specifying turnover frequencies of 355 and 3185Hz. The disc is similar to FOM2 except that after the initial 20sec of 1kHz there is 10k, 9k, 8k, 7k, 6k, 5k, 4k, 3k, 2k, 1k, 500, 200, 50Hz followed by another 10sec of 1kHz.
LPs were used by the BBC to send programmes to broadcasters in the Commonwealth and elsewhere. It was possible to record 30 mins on each side (for example two episodes of The Goon Show) and the shipping costs were far lower than using tape. I don't know whether this equalisation was continued after the introduction of the RIAA curve. It certainly did not appear as a switched option on any studio equipment I ever used.
AOM1 is a 12-inch diameter 'shellac' record. The label is 8 inches in diameter with strobe markings for 78 and 33 1/3-rpm. Both cuts are coarse groove and end with a concentric groove (sometimes called a 'locked groove') after a barely perceptible scroll. The strobe markings are only valid for a mains frequency of 50Hz when the record is viewed by artificial light fed from the mains. Alternatively the 1kHz tone could be compared for zero beat with a source of accurate 1kHz tone.
33 1/3-rpm coarse groove disks were widely used at one time. A 16-inch disk with a pitch of 120 grooves per inch could accommodate 15 mins and a 17¼-inch disc 15¾ mins thus a 30 min programme could be accommodated on two single-sided disks. As there was a drop in high frequencies towards the end of the disk, even with radius compensation, disks were cut alternately outside-in and inside-out so that there would not be a disturbing change in quality from one disk to the next. Minimum cutting diameter was 7½ inches.
16-inch coarse groove 'transcription' disks were widely used in the USA for programme distribution and, if processed, were made of vinyl. The quality was generally excellent. They had a signal to noise ratio of around 58dB but a directly recorded lacquer disk could achieve 68dB.
Next, an SOM3. On the modulated side are three piano pieces, unbanded and un-announced, lasting about 15mins and recorded at 33 1/3rpm.
But it's the reverse side that's worth a look!
left: A detail of the SOM3 reverse side.
Above: A MOM 1 which is 78rpm one side and 33/45rpm on the other.
Right: The DOM 25 was 78rpm both sides with 1K tone.
Below: A 1988 45rpm record, TG1, with test tones on one side and an orchestral sample on the other.
left: A more entertaining test record was 'The Teddy Bears' Picnic' a copy of which could be found in many studios. It was probably chosen because it had, for that time, a wide range of frequencies and was useful for a quick aural check that all was well. Here's an edited version:
Finally, not a record but a card stroboscope the purpose of which is clear from the text!
The instructions, below right, are printed on the reverse.
I have kept to the BBC convention that a direct recording was a 'disk' (not disc) and that a 'commercial gramophone record' (CGR) was a 'record'. 'Disc' was treated as a colloquial word for a CGR and its use was deprecated within the BBC. I don't recall that anybody took any notice of this except in staff instructions and training manuals. - B.T.