From: utzoo!decvax!harpo!eagle!karn
Newsgroups: net.video
Title: Mattel PlayCable
Article-I.D.: eagle.323
Posted: Fri May 28 23:39:58 1982
Received: Sun May 30 00:59:25 1982

I put a scope on the discriminator output of my FM tuner this
evening and examined the PlayCable channels.  (The signals from the
receiver audio outputs were distorted and unusable, due to the high bit
rate and the filtering of the stereo multiplex circuits).

The modulation appears to be simple Manchester phase encoding, at a
rate of 14,000 bits/sec.  There is always a transition every 71.4
microseconds, and optionally one halfway between, depending on the
data bit value.

The FM deviation appears to be at least the full 75 khz that FM
broadcast stations use.

Any speculation as to the higher level protocols? (e.g., bit ordering,
frame format, etc?)

Phil Karn