poynton@vector.Sun.COM (Charles Poynton) (03/11/89)
HDTV--Notes on Square Pixels for 1125-Line HDTV (TN28)
Charles A. Poynton <poynton@sun.COM>
Sun Microsystems, Inc.
89/03/09 11:40
SCOPE
This document outlines possible choices for detailed parameters (in
particular, sampling frequency) for the proposed 1125-line HDTV system
which, in addition to a picture aspect ratio of 16:9, achieve a sample aspect
ratio of exactly unity (i.e. "square pixels", in computer graphics
terminology).
The property of samples having equal horizontal and vertical spacing will be
important in HDTV applications related to computer graphics, and could
contribute greatly to the acceptance of the detailed HDTV parameters in
systems for printing and publishing, medical applications, simulator
applications, and other non-broadcast industries.
This document is an update of Poynton Vector Corporation Technical Note 28
dated June 27, 1987, which was submitted to the SMPTE AHG-HDSS
committee as document N15.4/6 REF 34 REV 7. General information about
HDTV is available in an associated document "Current State of High
Definition Television".
GENERAL
The basic parameters of the 1125-line HDTV system are now established in
the U.S. by ANSI/SMPTE 240M, and are contained in Annex II of the
international document CCIR Report 801-2:
Number of scanning lines: 1125
Number of active lines: 1035
Field rate: 60.00 Hz
Interlace: 2:1
Aspect ratio: 16:9
Samples per active line: 1920 for luminance (Y),
960 for colour difference (U, V)
With agreement on these basic analog interface parameters, the digital
representation of the 1125-line system is in the process of being
standardized.
"ACTIVE" SAMPLES
Note that although a line rate of 33.75 kHz is implied by these parameters
(1125x60 Hz), and the count of "active" samples per line is directly
specified, neither the total sample count per line or the sampling frequency
are specified or implied, and discussion of values for these parameters is
currently taking place.
The figure of 1920 "active" digital samples is suggested in an appendix of
ANSI/SMPTE 240M, and has agreement of the Japanese manufacturers' group
BTA. This number is derived from the number of "active" samples in CCIR
Rec. 601-1 (720), times the nominal doubling of horizontal resolution (2),
times the increase in aspect ratio (4/3).
The word "active" is in quotes because there is not agreement on whether
this number refers to the number of samples per picture width (i.e. between
the 50% points of a white flatfield), or the number of digital samples per
line which are permitted to be above blanking level. In a system with finite
bandwidth (and risetime), some number of non-blanking digital words are
required to represent the tails of the blanking transitions. If the tails of
blanking are truncated, then the picture width will be narrowed as the
signal passes through successive pieces of equipment.
CCIR Rec. 601-1 specifies the number of "active" samples per line as 720,
for both 525-line and 625-line television systems. The definition of
"active" is not made explicit in this document, but it is stated that the
number 720 was chosen as being sufficient to accommodate all non-blanking
samples at the widest picture width tolerance of both 525- and 625-line
systems. The reference number of samples per picture width (i.e. the
number of samples between the 50% points of a reference white flatfield) is
somewhat less than 720, nominally 702 in 625-line systems (for blanking
of 12 us), and between 702 and 712 in 525-line systems (for blanking
between 11.6 us and 10.8 us).
CHOICE OF 1920 "ACTIVE" SAMPLES
The BTA group has indicated the desire to maximize the count of samples
per picture width. The choice of 1920 samples per picture width has a
particular advantage when combined with a sample rate of 74.25 MHz: this
sample count represents an increase in picture width over existing (5:3
aspect ratio) HDTV hardware and software which is almost exactly equal to
the increase in aspect ratio from 5:3 to 16:9. With this choice of analog
picture width, existing 5:3 hardware and recorded material can be used in a
16:9 system, providing a slight cropping of picture width (about 6%) and a
slight alteration of aspect ratio (less than 1%) is allowed.
The blanking width resulting from this choice is about 3.77 us.
It is not clear how the BTA proposal would deal with transition samples.
Either the number of "active" digital samples should be somewhat greater
than 1920, or the number of samples per picture width should be somewhat
less than this.
PICTURE ASPECT RATIO OF 1.85:1
A choice of picture aspect ratio of 1.85:1 would achieve precisely square
pixels with the currently-proposed sampling parameters (74.25 MHz, 1920
samples per picture width, picture time 48/55 of total line time). 1.85:1
is the most widespread aspect ratio standard for 35 mm motion picture
production and distribution.
This choice would require changing the 16:9 picture aspect ratio parameter
of CCIR Report 801 and ANSI/SMPTE 240M.
CHOICE OF 1840 SAMPLES PER PICTURE WIDTH
For a total picture aspect ratio of 16:9, the aspect ratio of each sample (for
a choice of 1920 samples per picture width) is 120:115, only 4% off-square.
That is, the horizontal spacing of samples is 4% less than the vertical
spacing between samples. This ratio must decrease slightly in order to
accommodate transition samples within 1920 "active" digital samples; this
will make the samples even closer to being square.
With a choice of 1840 samples per picture width, the sample aspect ratio
can be made exactly unity. (The value 1840 is simply the number of lines
per picture height of 1035, times the picture aspect ratio of 16/9.)
This choice of active samples can be made for a number of choices of total
samples per line (i.e. a number of choices of sampling rate).
CHOICE OF 2200 TOTAL SAMPLES
The current BTA proposed sampling rate is 74.25 MHz, which is 11/2 times
13.5 MHz. This corresponds to 2200 samples per total line. Square samples
are easily achieved with this sample rate, resulting in 1840 samples per
picture width and a blanking time of 4.85 us almost identical the blanking
time of current 5:3 equipment.
However, with square samples at this sample rate a 4% picture aspect ratio
error would result for existing 5:3 material displayed on new equipment.
(1840 samples per picture width at 16:9 picture aspect ratio corresponds to
1725 samples at 5:3 aspect ratio.)
CHOICE OF 2070 TOTAL SAMPLES
A total sample count of 2070 simultaneously achieves square samples and
exact preservation of the aspect ratio of existing recorded material. This
sample count corresponds to a sampling rate of 69.8625 MHz, exactly 5.175
times the CCIR 601 sampling rate of 13.5 MHz. This is a somewhat
inconvenient multiple [23x3x3/(5x2x2x2)]. The corresponding analog
blanking value is about 3.29 us, which is somewhat short for current CRT
technology.
CHOICE OF 1053 LINES PER PICTURE HEIGHT
If the narrower of the 525-line and 625-line values for samples per picture
width is taken (625-line, at 702), then using the 2:1 horizontal sample
relationship outlined above, the number of samples per picture width
appropriate for the 1125-line system is 1872. With this count of samples
per picture width, a count of 1053 lines per picture width obtains samples
of exactly unity aspect ratio.
This option is unappealing in that it would require a change to the SMPTE
240M analog interface standard.
CHOICE OF 2100 TOTAL SAMPLES
A choice of 2100 total samples per line results in a sample rate of 70.875
MHz, 5.25 (21/4) times 13.5 MHz. This rate has the following properties:
1. downconversion to 525-or 625-lines can use a horizontal subsampling
ratio of exactly 1/2;
2. blanking time (3.67 us) is almost identical to the BTA proposal;
3. existing recorded material is reproduced with less than 1.4% aspect
ratio error, an error generally agreed to be imperceptible;
4. the sample rate is related to 13.5 MHz by the simple integer ratio
21/4;
5. the 240M sync waveform can be generated exactly, using a pulse
width basis of 21 clocks (the sync pulses can be generated exactly
using 23.625, 13.5, or 10.125 MHz clocks); and
6. samples have exactly unity aspect ratio.
This choice seems to be the best compromise between exactly square
samples, preservation of existing material, and blanking time.
RECOMMENDATION
Sun recommends:
1. that the term "samples per total line" (S/TL) be defined as the number
of sample periods per line period, and that this parameter be assigned
the value 2100;
2. that the term "luminance samples per active line" (S/AL) be defined to
be the maximum number of samples which may be assigned a value
different from blanking level, and that this parameter be assigned the
value 1920;
3. that the term "luminance samples per picture width" (S/PW) be
defined to refer to the number of digital samples between the 50%
points of the picture width, as referenced to a white flatfield, and
that this parameter be assigned the value 1840;
4. that a note indicate that these sampling parameters result in a
reference sampling frequency of 70.875 MHz, and a sample aspect ratio
of exactly unity.
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