e. PCM audio channel
The tape spares P C M area for the additional 2-channel au
dio information by wrapping itself over the D r u m by 220°
while the conventinal VCR tape wraps the D r u m a little
larger t h a n 180°. P C M audio signal is recorded through
Video-heads. In the conventional V C R s , B-head starts
scanning when A-head approaches the end of A-track. In
t h e 8mm V C R B-head starts recording P C M signal when
A-head traces the point a r o u n d 35° before it leaves the
D r u m . T h e sound information is compressed in this area
a n d expanded in playback. T h e specifications are a little
different from those of C o m p a c t Disc.
Video head A
Drum
Plays PCM tracks
PCM tracks not used
A-head
Tape direction
Video area
B-head
(a) 1 / 2 " VCR
A-head
(b) 8 m m VCR
ing memory space and increasing recording density. The
signal is processed by the cross-interleave system. Then it
is bi-phase-modulated and applied to a recording ampli
fier. The bit rate is 5 . 8 M b / s . 90dB dynamic range can be
achieved with only 8-bit d a t a . T h e sampling frequency is
31.5kHz in N T S C (31.25kHz in P A L ) .
Data RAM
Bi-PHASE
Error d e t e c t i o n , cross inter
leave, correction and inter
polation
MOD
DEM
Fig. 4 2 PCM Block Diagram
8 m m
CD
Sampling f r e q .
3 1 . 5 K H z
4 4 . 1 KHz
Freq. response
2 0 - 1 5KHz
2 0 - 20KHz
Quantization
10-bit/ch
16-bit/ch
1 0 - 8
8-> 14 modulation
(EFM)
Recorded signal
Biphase m a r k *
N R Z I *
Error correction
Cross Interleave
CIRC*
Table 6 PCM Standards of 8mm VCR and Compact
Disc
* CIRC
Refer to Compact Disc, Vol 7.
* Bi-phase-mark
This signal inverts at every border of bits and in the cen
ter of "1" while NRZI inverts only in the center of "1
Synchronizing is easy with this signal because the clock
component always appears. The frequency bandwidth is
limited; from transfer clock frequency to its double. This
has been determined to be in the same band of the video
signal to make the Heads compatible to 8mm VCR and
PCMACR.
Fig. 4 3 BIPHASE MARK
Tape
1 8 0
° + a
2 2 1 °
Video head B
Fig. 41 Extended Rapping Angle of 8mm VCR
Tape direction
Video area
180°
180°
B-head
PCM area
The P C M signal can be dubbed, or recorded independent
ly, while 1/2" Hi-fi VCRs are incapable of dubbing F M
audio signal and have a problem of head switching audio
noise. P C M circuit block has an analog noise reduction
circuit.
10-bit A/D-converted data are compressed to 8-bit for sav-
Compression
expansion
Input
O u t p u t
Analog
N.R.
lObit
A—D
A—D
10-»8
1 0 - 8
8 bit
REC head
PB head
NRZI
Bi-phase mark
NRZ
5 0
