Pioneer tuning fork Service Manual page 52

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 VCRs, B-head starts
scanning when A-head approaches the end of A-track. In
the 8mm V C R B-head starts recording P C M signal when
A-head traces the point around 35° before it leaves the
D r u m . The sound information is compressed in this area
and expanded in playback. The specifications are a little
different from those of Compact Disc.
Tape direction
Video 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-
Input Analog
N.R.
O u t p u t
Sampling f r e q . 31.5KHz
2 0 - 1 5KHz
Freq. response
Quantization 10-bit/ch
1 0 - 8
Recorded signal Biphase m a r k *
Error correction Cross Interleave
Table 6 PCM Standards of 8mm VCR and Compact
Disc
* CIRC
Refer to Compact Disc, Vol
5 0
A-head
B-head
(a) 1 / 2 " VCR
Compression
expansion
lObit
A—D 10-»8
1 0 - 8
A—D
Fig. 4 2 PCM Block Diagram
8 m m CD
4 4 . 1 KHz
2 0 - 20KHz
16-bit/ch
8-> 14 modulation
(EFM)
N R Z I *
CIRC*
7.
Video head A
1 8 0 ° + a
2 2 1 °
Fig. 41 Extended Rapping Angle of 8mm VCR
Tape direction
Video area
180°
PCM area
(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 5.8Mb/s. 90dB dynamic range can be
achieved with only 8-bit data. The sampling frequency is
31.5kHz in N T S C (31.25kHz in P A L ) .
Data RAM
8 bit
Error detection, cross inter­
leave, correction and inter­
polation
* Bi-phase-mark
This signal inverts at every border of bits and in the cen­
ter of "1" while NRZI
Synchronizing is easy with this signal because the clock
component always appears.
limited; from transfer clock frequency
has been determined
signal to make the Heads compatible
PCMACR.
Fig. 4 3 BIPHASE MARK
Drum
Tape
Plays PCM tracks
PCM tracks not used
Video head B
A-head
180°
B-head
Bi-PHASE
REC head
MOD
DEM PB head
inverts only in the center of "1
The frequency bandwidth
to its double.
to be in the same band of the video
to 8mm VCR and
NRZ
Bi-phase mark
NRZI
is
This