[comp.graphics] HDTV--Notes on Square Pixels for 1125-Line HDTV

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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