High com

High Com (also HIGH COM , written with half a space) is an analog noise reduction system that the Telefunken company developed in the 1970s.

It was available from 1978 in some higher quality cassette recorders (first from Telefunken, then also from other manufacturers) and achieved significantly better noise suppression than the Dolby B method that was widespread at the time (max. 20 dB with High Com compared with max. 9 dB with Dolby B). The system was also offered by Telefunken as an external solution, switchable between the cassette deck and the amplifier .

Mode of action

The better noise suppression compared to Dolby B was based on the design as a broadband compander , i.e. the inclusion of the entire audible frequency range instead of just the high frequencies.

High Com is based on the experiences that Telefunken has made with the telcom c4 developed for professional recording technology . It was a multi-band compander, in which a compressor / expander was responsible for a frequency range via a 4-band crossover.

advantages

In fact, high-com recordings in quiet passages were audibly lower in noise than conventional recordings. Compared to the then competing Dolby-B process, the following advantages were also often mentioned:

• much better noise reduction (see above)
• good effect even with low-frequency interference
• Interchangeability, d. H. recording on device A and playback on device B posed no problems
• less altitude attenuation with azimuth problems
• No presence attenuation in the case of azimuth problems and altitude overload
• Can also be used if the recording was carried out entirely without a noise reduction system and the playback device enables High Com to be switched on manually, or with an external High Com device (e.g. Telefunken CN750)

disadvantage

According to some user reports, critical pieces of music (piano music, solo instruments without accompaniment) could hear a strong "noise pumping", since with the dynamic expansion during playback, the noise of the cassette tape can also be heard alternately because in the cases mentioned it is not different from the Useful signal is covered. Some people found these effects particularly annoying. On the other hand, the noise pumping could not be confirmed by every user, on the contrary: According to some reports, "pumping" only occurs with older high-com versions or with a wrong calibration.

The Dolby B noise suppression has advantages in terms of background noise (pumps, flags) thanks to the sliding band function. Loud solo instruments (especially basses) that suddenly set in can sometimes sound significantly weakened with High Com. These effects were even more pronounced with the competing noise suppression systems dbx and adres (from Toshiba ).

Improvements to the high-com system

Over the years there have been several improvements to Telefunken's high-com chip. The first systems could only record and play back with High Com, but not with Dolby B. This meant that cassettes recorded with Dolby B could only be used to a limited extent. The second high-com generation had an integrated process called DNR (Dynamic Noise Reduction), with which one could also adequately play Dolby B-coded cassettes (there was an expander compatible with Dolby B). The third high-com generation, used by Akai, but also to be found in the Telefunken RC200 and RC300, was even able to record with DNR. A kind of Dolby B was integrated, which even worked without loss of height (better than almost all Dolby B systems).

Around 1979, Telefunken published publications on further developments of the system as a 2-band and 3-band compander based on the chain amplifier principle under the names High Com II and High Com III.

The High Com II system was launched on the market from 1979/1980 by the Nakamichi company under the name Nakamichi High Com II Noise Reduction System in the form of external compander units. In this form, the system, which can be connected to any stereo tape recorder and cassette recorder, achieved a dynamic gain of around 25 dB. For each of the two stereo channels, two high-com modules of the type TFK U401B were used, each of which acted on different frequency bands through a crossover, which led to significantly reduced breathing noise and fewer artifacts. At the same time, the method was hardly susceptible to interference from drop-out errors or greater high-frequency attenuation due to azimuth errors . By means of a built-in test tone generator, level controls and level indicators, the system could be optimally adapted to the respective recording and playback devices and the tape material used, similar to the play-trim control of later Dolby devices (see section historical development ). The disadvantage of the complex implementation was that two such devices were required for the rear belt control, so that the system could not be widely used because of the high costs.

Telefunken has also made several improvements to the external circuit over the years, initially by using other timing elements for a longer decay time constant in order to improve the distortion in the bass range, and later by adding some transistors and passive components. The Funkschau 26/1982 writes about this: “In this case, the system modification does not start with the dimensioning of the time values, but the temporal transition of the decaying control voltage is optimized. In the previous system design, when the rapid level drop was detected (after the hold time had elapsed), the long decay time constant was abruptly switched to the short value. As a result, relatively large changes in gain occurred just a few milliseconds after this process. This resulted in the risk of audible dynamic errors (sometimes also referred to as 'pumping') in the event of 'drop-outs' or more severe 'mistracking'. The system modification results in a smooth transition for the change from long to short time constant mentioned. This change is also referred to as rounding or 'soft switching' and brings about a significant calming of the dynamic system behavior, so that the previously described conditions are completely masked in practice even under critical signal and tolerance conditions. "

Historical development

In the first few years there was a real high-com euphoria, as the new system promised for the first time practically noise-free sound recordings on the compact cassette, which was widespread at the time and was quite noisy . The well-known WDR television program Hobbythek even brought out a "HobbyCom" kit in the early 1980s, a small do-it-yourself device equipped with original Telefunken circuit boards, with which conventional cassette recorders can enjoy the high- Com noise reduction came.

In the meantime, the introduction of a special variant of Telefunken's "High Com FM" (with reduced compression) for German VHF radio was planned, in which the signal was to be compressed on the transmitter side and expanded on the receiver side. The receiver could have recognized compressed transmissions from an identifier modulated onto the stereo pilot tone (similar to traffic radio) and could have switched on the expander if necessary. Due to the special adaptation, eavesdropping would have been possible with good quality without expansion. But this system was never introduced.

Even if in the meantime many manufacturers brought cassette devices with High Com onto the market, it could not ultimately prevail.

Possible reasons for this:

• High-com recordings absolutely require a high-com cassette player for playback; playback without the necessary high-com equalization sounds unreasonably bad. The situation is different with the competing Dolby B: playback without Dolby B equalization does without the noise reduction, but sounds acceptable and only a little bit treble - which was even favorable to cheap playback devices.
• The first high-com cassette devices did not have Dolby B, so Dolby B recordings could not be played adequately on the new devices. This disadvantage was soon resolved.
• Probably the most important reason for the failure of High Com was the introduction of Dolby C in 1980. Compared to Dolby B, Dolby C offers significantly better noise suppression (approx. 16 dB), with the undesirable side effects ("noise pumping", dynamic distortion) being significantly smaller than were at High Com. All Dolby-C recorders also had a switch for Dolby B. Previous recordings could therefore continue to be listened to without any problems (which also worked with the DNR mentioned above).
• In many cases, the newer Dolby C - like its predecessor - still caused treble distortion, ie recordings with High Com sometimes sounded better than those with Dolby C. However, Telefunken's license conditions and costs may be too restrictive or the advertising budget was too tight low in order to assert itself economically against the globally operating Dolby laboratories. Dolby C also put the end of a kind of compander war. In addition to Telefunken's High Com, Sanyo's SuperD, Toshiba's Adres and dbx vied for buyers' favor. Dolby founder Ray Dolby , who initially saw no need to develop a better system than his Dolby B, was convinced by the - mostly Japanese - manufacturers of cassette recorders that the time was ripe for better noise suppression. When developing Dolby C, he was able to study the weaknesses of the compandering compander and avoid them in his system.

Finally, from today's perspective, there is another disadvantage of High Com: In older High Com recordings, the magnetization of which has suffered due to storage, the resulting errors are amplified by the strong equalization. As a result, old high-com recordings can often no longer be played adequately after long periods of storage. This problem also exists with Dolby B and C, but NAD and Dolby developed the so-called play trim control for these systems , with the help of which a high frequency response that is incorrect on the band side can be corrected before dynamic expansion. With Nakamichi's High Com II, this was not a problem, as the compander could be adjusted by the user afterwards. A calibration tone recorded by the compander at the beginning of the tape when required was very useful for the subsequent adjustment of the playback chain.

The last high-com cassette recorders were sold around the end of 1986.

Cassette recorders with high com

• Akai GX-F37
• ASC AS-3000
• Audion D 700
• Blaupunkt XC-240, XC-1400
• ELIN Professional Micro Component Cassette Deck - Model TC-97
• Eumig FL-1000 µP
• FILTRONIC FSK 200
• Grundig MCF 200, MCF 600, CF 5100, SCF 6200
• Hitachi D-E75
• Imperial td 6100
• Intel Professional Micro Component Cassette Deck - Model TC-97
• Körting C 102, C 220
• Nikko ND-500H
• nippon TD-3003
• Palladium Mico Line 2000C
• Saba CD 278, CD 362
• Schneider SL 7270 C
• Sencor SD-6650
• Siemens RC 333, RC 300
• Studer A710
• Telefunken, various devices
• Tensai TFL-812
• Uher CG 321, CG 344, CG 356, mini-hit
• Universum Ct 2307A, CT 2318 (SYSTEM HIFI 7500 SL), CT 2337
• Wangine K-3M, WSK-220

literature

• Thomas Görne, Ulrich Schmidt (Ed.): Sound engineering. 1st edition. Carl Hanser, Leipzig 2006, ISBN 3-446-40198-9 .
• Jürgen Wermuth: Dynamic expansion through a new kind of studio compander. 10th Tonmeistertagung Cologne, 19. – 22. November 1975.
• J. Wermuth, St. F. Temmer: Dynamic expansion through a new kind of studio compander. In: Funkschau. No. 18, 1975, p. 571 ff.
• E. Schröder, J. Wermuth: A new compander system - basics and possible uses. Lecture at the FKTG conference in Freiburg, October 5, 1976, published in: FKT. 30, No. 12, 1976, pp. 9-11.
• CR: Kompander improves magnetic copying. In: radio mentor. No. 4, 1965, pp. 301-303.
• Gerhard Dickopp, Ernst Schröder: The Telefunken Compander. In: Rundfunktechnische Mitteilungen. Vol 22, Issue 2, 1978, pp. 63-74.
• Gerhard Dickopp, Ernst Schröder: Measurement method for compander. In: Funkschau. Issue 17, 1978, pp. 29-32.
• HIGH COM. In: Audio Engineering. 3, 1981, pp. 30-35.
• Testing of a modified HIGH COM compander for use in RF transmission in VHF radio. (= IRT Technical Report. 55/81). December 30, 1981.

Individual evidence

1. ↑ See also high-com modifications ( memento of the original from October 8, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
2. ↑ Vintage cassette ( Memento of the original from December 4, 2008 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. 

References

• Ernst F. Schröder: The story of HIGH COM. A developer at Telefunken tells
• U401BR, AEG-Telefunken Semiconductor Information 2.80