There's   FRED document source, a FRED manual by name from 1972 forward, which I point to from my "System 00" Web page:

http://www.retrotechnology.com/memship/cosmac_system_00.html#FREDmanual

http://www.retrotechnology.com/memship/Sarnoff_FRED_manual_Weisbecker_Robbi.pdf

Here's what it says on "Cassette Player". However, this is a misdirection - it's about a 1972 non-microprocessor system, it predates any of the "FRED 2's" which have a single or dual microprocessor-chip oprating them; and of course the VIP's. 

"A 4.3kc tone recorded on tape causes the tape to stop automatically at the end of the tone. This feature permits the tape to be divided into "FRAMES" which can be sensed by program. Another mode of tape operation permits data or program bytes to be read from tape and stored sequentially in memory.

"Recording is bit-serial each byte on tape is represent by 10 bits. A start bit (S) is always "1". this is followed by the 8 byte bits (0-7). A parity bit (P) makes the 10 bits even. Parity and start bits are used by the interface circuits and dropped priot to storing each byte in memory. Each byte from tape is stored sequentially in memory via the direct memory channel (cycle stealing).

Each bit on tape is represented by a 5ms tone burst. A 5.2kc tone represents "0" and a 6.2kc tone represents "1". Tones are separated by 5ms gaps. This results in bits which are self-clocking. Recording density is on the order of 50 bits/inch. Data is loaded in memory at a 10 byte/second rate. The low packing density and self-clocking features permit reliable operation with standard audio cassettes." - end quotes

A figure number 8, shows the cassette format. It draws bits as bursts of tones just as described in the text. Simple math says 5200HZ is .1923ms, so a 5ms burst would be 26 cycles! Likewise 6200Hz becomes 31 cycles. If anyone sees tapes like *these*, these must be "system 00" or "pre-microprocessor FRED" tapes.

The schematics for the cassette interface, are as follows: "LOGIC-17" shows three LM567 tone-decoders. They are marked "READ 1" and "6000", "READ 0" and "5000", and "READ S" (stop) and "4000".