OIRT VHF is the Eastern European VHF broadcasting standard (“Soviet VHF”). The frequency range is from 65.9 MHz to 74 MHz, used for radio broadcasting in the former USSR, Eastern European countries, and Mongolia. At present, it continues to be used in some cities in parallel with the VHF CCIR band.
The OIRT VHF broadcasting frequencies were allocated within television frequency ranges, in the gap between television channels. In Russia, the frequency range for VHF radio broadcasting is located between the second and third television channels.
In the “Soviet VHF” standard, unlike VHF CCIR, the channel bandwidth was originally different (according to OIRT, the television signal occupies 8 MHz, while in CCIR it is 7 MHz). The standards also differ in the method of stereo signal encoding and the ability to embed an RDS signal (OIRT VHF does not provide for text data transmission via an RDS subcarrier). For stereo program transmission, OIRT VHF uses polar modulation.
Other quality indicators of both systems are practically the same:
- transmitted audio frequency band: 30–15,000 Hz;
- stereo channel separation — at least 25 dB (40 dB or more in modern models);
- degradation of signal-to-noise ratio in mono reception: 2 dB (Soviet)/1 dB (Western).
The degradation of the signal-to-noise ratio in mono reception is caused by the fact that part of the transmitter power in both systems is used to transmit the additional (L-R) signal. Due to the lower frequency of the difference signal subcarrier (31.25 kHz), the OIRT VHF system has a narrower spectrum of the composite stereo signal (165 kHz compared to 190 kHz in the American system), which theoretically allows more stations to be placed in the VHF band and improves transmitter power usage. However, in the Soviet system, the gap between the upper boundary of the tonal spectrum (15 kHz) and the lower sideband edge of the modulated subcarrier (31.25 kHz – 15 kHz = just over 1 kHz) is very small, imposing strict requirements on filter circuits for harmonic suppression. In the pilot-tone system, this gap is 8 kHz, which makes it easy to filter tonal and harmonic signals. Therefore, the probability of crosstalk in stereo decoders of the system with polar modulation is significantly higher.
Initially, to receive VHF CCIR (FM) stations with Soviet-standard receivers, FM-VHF converters were used. Such adapters were installed on the antenna (or inline with the antenna cable) of a VHF receiver and added the difference signal. As a result, old Soviet radios and tape recorders could receive FM band stations.
Related articles:
FM,
VHF CCIR,
Frequencies of digital TV channels in Moscow,
Polar modulation,
Frequencies of television channels with subcarriers,
Frequencies of FM radio stations in Moscow