The Soviet Union and its East European allies used six types of the jamming audio signals:
1. To block out the "most anti-Soviet" stations, a wide spectrum electronically generated noise signal was used. RFE/RL, Voice of Israel, and Radio Tirana would experience this type of jamming.
2. On August 3, 1964, one more source of interference was invented - Radio Mayak program, transmitted in FM mode and heard distorted on domestic receivers - to jam some "grey propaganda" stations such as VOA, BBC, Deutsche Welle, and R. Beijing.
3. In 1976, Soviets started to use the speech resembling signal. Its advantage was that it conformed to the timbre of the human voice. This jamming sound, which used to be played back from open reel tapes, was composed of two voices of male and female Russian announcers.
4. A unique case was the Polish service of RFE/RL: from 1971 until 1980 only light instrumental music was employed to jam it, both in clear AM and distorted FM modes.
5. East Germany aired its domestic radio programs via medium wave transmitters tuned in to several hundred Herz outside of the RIAS frequencies.
6. Czechoslovakia used the swinging carrier, also known as wobler, AM transmitters to jam RFE/RL.
"Radio Jamming in the Soviet Union, Poland and others East European Countries" by Rimantas Pleikys.