m m 1SFS/6
THE 0UEEN5 AWARD
RESEARCH DEPARTMENT REPORT
Epipment for the transfer of
digital video signals to paper tape
C. K.P.Clarke
Research Department, Engineering Division
THE BRITISH BROADCASTING CORPORATION June 1973 (PH-108)
BBC RD 1973/6
UDC 621.376.56
621.397
681.3.074
EQUIPMENT FOR THE TRANSFER OF DIGITAL VIDEO SIGNALS TO PAPER TAPE
C.K.P. Clarke
Summary
A digital computer can be used to simulate processing methods for the bit-rate
reduction of digital television. To assess such methods, television signals must be con-
verted to a form compatible with computer data-handling systems. This report describes
equipment for transferring monochrome pulse-code modulation television signals derived
from a slide scanner to punched paper tape. One eight-bit word is extracted from the
repeated signal during each picture period and is then punched as a single eight-element
character on the tape.
The equipment has been used to produce two paper tapes each containing an
area of picture 256 picture elements wide and 256 lines high. The report includes a
picture reconstituted from one of the tapes.
Because the phase of the colour subcarrier at a point in the picture is the same
only once every four picture periods, the equipment must be modified to take samples at
this rate, if it is to be used with coded colour signals. More extensive modification would
be needed if subcarrier-locked sampling were to be used.
Issued under the authority of
Oa^^
Research Department, Engineering Division,
BRITISH BROADCASTING CORPORATION Head of Research Department
(PH-108)
BBC RD 1973/6
EQUIPMENT FOR THE TRANSFER OF DIGITAL VIDEO SIGNALS TO PAPER TAPE
Section Title Page
Summary Title Page
1. Introduction 1
2. Principle of operation 1
3. Method of testing . . . , ; 3
4. Modifications for colour signals 3
5. Conclusions 3
6. Reference , 3
(PH-108)
June 1973
BBC RD 1973/6
UDC 621.376.56
621.397
681.3.074
EQUIPMENT FOR THE TRANSFER OF DIGITAL VIDEO SIGNALS TO PAPER TAPE
C.K.P. Clarke
1. Introduction
A general-purpose digital computer may be programmed
to perform exactly the same arithmetical and logical
operations as a processor for the bit-rate reduction of
digital television. However, such computers are not
sufficiently fast to accept and process television signals in
real-time. So, in order to simulate a bit-rate reduction
method by using a digital computer, it is necessary to
reduce the input data rate by converting the signal to a
form which may be readily accepted by a computer, for
example, as characters punched on paper tape. Also, the
signals processed by the computer must be stored and sub-
sequently read out at television rate onto a picture
monitor.
slide
scanner
monochrome
video
a.d.c.
mixed
syncs
line locked
clocks
sprocket punch
and feed
8-bit p.c.m
video
v:
sompler
\y
8-blt sample
words
paper tape
punch
In this way, many of the degrading effects introduced
by the bit-rate reduction process may be seen and assessed
without building a specialised processor. However, because
it is impracticable to provide sufficient storage to hold
several frames of processed signal, it is not possible to
assess the effects produced on moving pictures with this
simulation method.
Fig. 1 - Arrangement for transferring digital video signals
to paper tape
a further counter, advanced once per picture, is used to
register the number of samples taken. The outputs of these
counters are compared as shown in Fig. 2 and a sample is
taken when the outputs are identical. As a result, the 2'*
To provide suitable input data, equipment has been
developed to transfer eight-bit pulse-code modulation tele-
vision signals to punched paper tape. The equipment
transfers a square section of picture consisting of 2'^
(65536) picture elements, that is approximately one-sixth
of the area of a television picture. With a punch operating
at 25 characters per second, the transfer takes 44 minutes
and uses 170 metres of paper tape.
2. Principle of operation
The rate of signal transfer to the paper-tape punch is
reduced by sampling a repeated picture signal. The arrange-
ment used Is shown in Fig. 1, where the slide scanner
provides stationary monochrome picture signals which are
converted to eight-bit digital form by the Analogue-to-
Digital Converter,^ and the sampler selects one word during
each picture period and operates the paper-tape punch to
produce a corresponding eight-hole character.
The sampler contains two counters which establish
a co-ordinate system for the picture. The horizontal
counter is advanced at 851 times line-frequency and reset at
the beginning of each line, whilst the vertical counter runs
at line rate and is reset at the beginning of each field. A
picture-rate square wave is used to distinguish the fields and
odd/even field
square wave
horizontal
counter
LSB >'
— 8-bits---
>' MSB
vertical
counter
LSB \f
--7-bit5---
>'MSB
comparator
LSB
-16-bils
sampling
pulse
MSB
sample counter
Fig. 2 - Generation of sample position
(PH-108)
■s
.o
.til
BBC RD 1973/6
- 2
states of the sample counter correspond to 65536 different
positions on the picture. By comparing bits 1 to 8 of the
sample counter with the horizontal counter, bit 9 with the
Odd/Even-field square wave and bits 10 to 16 with the
vertical counter, an area 256 picture elements wide and 256
lines high is canned sequentially. A sequential scan was
chosen in preference to an interlaced scan to minimise the
effect of drift in the analogue circuits of the slide scanner
and analogue-to-digital converter during the scanning period.
The full logic diagram of the sampler is shown in Fig. 3.
Whilst scanning along any line, the interval between
sampling pulses is two field periods. However, because a
sequential scanning order has been chosen, the interval
between the last sample of a line and the first sample of the
next line varies from just over one field period to just over
three field periods. In order to provide a regular flow of
data to the punch, the eight-bit words must be stored in a
second register at the end of each picture and read from
this to the punch.
Although the horizontal counter tal<es up 851 dif-
ferent states in a line period, samples are only taken over
the 256 lowest states. By resetting the counter to an
appropriate state at the beginning of each line, the sampled
area can be positioned anywhere along a line. A similar
method is used to determine the vertical position of the
sampled area.
Operation of the sampler is controlled by a switch
having two positions: Run and Reset. The switch is
latched to ensure correct starting and the latching circuits
automatically stop the sampler after 2'* samples have been
taken.
The system produces paper tapes on which the data
appears as a continuous block of 2'* characters. Since
samples are only taken during the active line, there is no
possibility of null characters (no holes punched) being
produced as data and the section of lead-in tape can be
recognised.
3. Method of testing
Although it was not possible to check the picture
information stored on the tapes directly, tests were made
to establish that the correct samples were punched. The
sampling order was checked by observing the traverse of the
sampling pulse on a picture monitor. Paper tapes made
from analogue waveforms of known amplitude and timing
were examined visually to check that the correct sample
values were transferred; further, an electronic counter was
used to count the number of characters punched on full-
length tapes. No errors were detected.
Two paper tapes of picture signals have been pro-
duced, one corresponding to a portrait and the other to a
section of test card. The portrait comprises both plain
areas and areas of fine detail, whilst the test card includes
horizontal, vertical and diagonal bars. Fig. 4 shows the
portrait* reproduced on a computer line-printer using a
sixteen level grey-scale of variable character density.
4. Modifications for colour signals
The equipment as described cannot satisfactorily be
used with coded colour signals because the phase of the
colour subcarrier at a point in the picture is the same only
once every four pictures. Thus, to reproduce a portion of
a coded colour signal, the samples must be taken from every
fourth picture. To prevent the transfer taking four times
as long, a store could be used to hold four consecutive
samples from one picture which would then be punched at
the rate of one per picture (the maximum rate at which the
paper-tape punch can operate).
If it were intended to decode the colour signals as part
of the computer processing, it would be much more con-
venient to use samples phase-locked to the subcarrier than
the line-locked samples used for monochrome transfers.
However, subcarrier-locked sampling would require a con-
siderably more complicated horizontal counter because line
pulses could no longer be used for resetting the count.
Furthermore, extra samples, conveying the subcarrier
reference phase on each line, would be required.
5. Conclusions
The monochrome transfer equipment establishes a
system of positional co-ordinates for a television picture
851 picture elements wide and 625 lines high. Any area of
the picture 256 picture elements wide and 256 lines high
can be scanned sequentially and this section of the picture
signal transferred in digital form onto punched paper tape.
Each picture element, represented digitally as an eight-bit
word, is punched as a single eight-digit character on the
tape. Two paper tapes of picture signals suitable for testing
bit-rate reduction techniques have been made, one describ-
ing a portrait and the other a section of test card.
The design of the equipment could be easily modified
to accommodate coded colour signals provided that samp-
ling were still locked to a multiple of line frequency. How-
ever, to transfer coded colour signals in a convenient form
for subsequent decoding, it would be necessary to use
sampling locked to the colour subcarrier; this would entail
extensive changes to the counter determining the horizontal
co-ordinate.
6. Reference
1. Pulse code modulation of video signals: 8-bit coder
and decoder. BBC Research Department Report No.
1970/25.
• Reproduced by kind permission of Professor P.S. Brandon,
Cambridge University.
BBC RD 1973/6
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