Toshiba D-VR3SU Service Manual page 132

Circuit Operating Descriptions
(3) Chroma Signal Recording System
VIDEO 1N
28
REC LUMINANCE
1) Outline
Fig. 7-20 shows the chroma signal recording system. The chroma signal recording process is performed by video
IC. The input video signal is fed to Pin 28 of IC and supplied to Y/C COMB circuit through AGC AMP.
The output signal of Y/C COMB circuit is applied to ACC amplifier. The ACC amplifier is used for both burst
ACC which keeps the burst level at a constant value in recording and the color ACC which controls the reference
level of the burst ACC with the color signal level. The color ACC works to maintain a relatively high output level
by boosting low level input signals to improve color S/N ratio. The signal is then applied to the burst emphasis
circuit. Burst emphasis emphasizes the burst signal by +6dB during recording and feeds it to the main converter.
The 3.58MHz signal are mixed in the main converter to perform frequency conversion.
The main converter is a mixer having the two types of output components which are the added frequency of
4.21+3.58=7.8MHz and the difference frequency component 629KHz.
Added frequency is rejected by the 1MHz LPF and the 629KHz down converted chroma signal is supplied to the
luma/chroma mixer of pre-amp block and then recorded on the tape via the record amp and heads.
AFC detection is performed with the head switching pulse and the fh signal generated from 321fh VOC output.
The detector output controls the VCO frequency which will be locked precisely at 320fh (5.035MHz).
he 320fh signal is counted down to 1/8 and the resultant 40fh (=629KHz)carrier signal is phase shifted triggered
by each horizontal sync signal which is wave shaped as a 50% duty pulse by the pulse generator.
The direction of the rotational phase shift depends on the levels of the rotary head switching signal from pin 57
and when the switching signal is "H" level, the phase is retarded by 90 degrees for every 1H, and when is is at a
"L" level it will advance by 90° for every 1H this 40fh phase shifted sub-carrier (PSSC) signal enters the sub-con-
verter and the 3.58MHz carrier signal is locked at the color burst frequency by the record APC.
The PSSC signal is frequency converted into 3.58MHz +/-629KHz. Then 4.21MHz component (=3.58MHz+/-
629KHz)is extracted through a 4.2MHz BPF.
The 4.21MHz signal is used as a carrier signal for down conversion of the color signal as described previously.
7-32
VIDEO
AGC
1MHz
LPF
+
69
Fig. 7-20 Chrominance Record Process
39
CCD
YNR Y/C
LPF
COMB
MAIN
BURST
CONVERTER
EMPHA
70
IC301 LA71207M
34
3.58
BPF
ACC
AMP
REC
CURRENT
AMP
73 SP H'D
66 SLP H'D