Understanding composite digital video receivers were built because the circuit complications would not yield any

visible picture improvements. It is surprising that SMPTE 244M was

developed using obsolete chrominance signals.
Over many years, video equipment manufacturers have responded to the
trend toward digital video by producing a large number of application- As Figure 4 illustrates, the sampling instants coincide with the peak
specific digital black boxes. These products were developed to fulfill positive and negative amplitudes of the I and Q subcarrier components.
specific production needs. They were operating at incompatible sample The upper part of the drawing shows that these sampling instants provide
rates, number of bits per sample and quantizing range, and so they are an adequate representation of the B-Y/R-Y information. Given a sampling
often incompatible with each other. The one thing they do have in common frequency fS = 14.3181 MHz (nominally 14.32 MHz) and a horizontal
is a link with their analog ancestry: analog composite I/O interconnect ports scanning frequency fH = 15,734.25 Hz, the number of samples per total line
that make them compatible with all-analog composite production studios. is equal to fS/fH = 910. The digital active line accommodates 768 samples.
The remaining 142 samples comprise the digital horizontal blanking
The continuing trend towards an all-digital studio resulted in the need for
interval.
digital video equipment industry standards. What resulted was the
development of two sets of composite digital standards for studio
equipment: the 4fSC NTSC standard and the 4fSC PAL standard. The quantizing range and its implications
Figure 5 shows the relationship between analog NTSC signal levels and
These standards specify that the analog composite video signal must be eight-bit and 10-bit sample values of a 100/7.5/100/7.5 color bars signal.
sampled at a rate of four times the color subcarrier frequency (4fSC). The The 10-bit approach provides 1024 digital levels (210), expressed in
number of bits per sample plays an important role in determining the signal decimal numbers varying from 000 to 3FF. Digital levels 000, 001, 002, 003
quality and the economics of videotape recording, so the standards allow a and 3FC, 3FD, 3FE, 3FF are protected and not permitted in the digital
choice of eight or 10 bits per sample. stream. This leaves 1016 digital levels, expressed in decimal numbers
varying from 4 to 1019, or in hexadecimal numbers varying from 004 to
In North America, the initial interest in 4fSC composite digital videotape 3FB, to represent the video signal. The sync tip is assigned the value 16
recorders (VTRs) was spurred by the need to replace obsolescent analog decimal or 010 hexadecimal. The highest signal level, corresponding to
composite VTRs with digital VTRs that had analog I/O ports. A number of yellow and cyan, is assigned the value of 972 decimal or 3CC
manufacturers developed such products, identified as D2 (Sony and hexadecimal. The standard allows a small amount of bottom headroom
Ampex) and D3 (Panasonic) digital VTRs. Subsequently, a wide range of (some call it footroom), levels 4 to 16 decimal or 004 to 010 hexadecimal,
compatible 4fSC digital video studio-quality equipment appeared on the and top headroom, levels 972 to 1019 decimal or 3CC to 3FB
American market. In Europe, however, interest in 4fSC VTRs was limited hexadecimal. The total headroom is on the order of one dB, and allows for
because these VTRs cannot handle SECAM. misadjusted or drifting analog input-signal levels. This reduces the signal-
to-RMS quantizing-error ratio (S/QRMS) by the same amount. The
This article discusses the sampling and quantizing characteristics that theoretical S/QRMS of a 4fSC device with analog I/O interfaces is given by the
govern these VTRs as specified in the SMPTE 244M standard. following formula:

General specifications S/QRMS(dB) = 6.02n + 10.8 + 10 log10 (fS/2fmax) - 20 log10[Vq/(VW-VB)]

The SMPTE 244M standard sets the sampling frequency at four times the where:
subcarrier frequency, or 14.3181 MHz (14.3 MHz nominal). The sampling n (number of bits per sample) = 10
clock is derived from the analog signal's color burst. Figure 1 shows the fS (sampling frequency) = 14.32 MHz
sampling spectrum of the 4fSC NTSC. The shaded area represents the fmax (maximum baseband frequency) = 4.2 MHz
suppressed subcarrier and its sidebands. In this example, the sidebands Vq (quantizing range) = 1.3042 V
are limited to ±600 kHz. The sideband bandwidth depends on the NTSC VW (white signal amplitude) - VB (blanking level) = 0.7143 V
encoder design. The SMPTE 170M standard allows narrow-bandwidth Given the above values, the calculated value of S/QRMS for a 10-bit system
(±600 kHz) and wide-bandwidth (±1.2 MHz) chrominance sidebands. is 68.10 dB.

There is a significant gap between 4.2 MHz, the maximum nominal NTSC In an eight-bit system, 254 of the 256 levels (01 through FE) are used to
baseband frequency, and 7.16 MHz, the Nyquist frequency. Unlike the ITU- express a quantized value. Levels 00 and FF are protected and not
R BT 601 component digital standard, SMPTE 244M does not specify the permitted in the data stream. The calculated theoretical value of S/QRMS for
characteristics of the anti-aliasing and reconstruction filters. The an eight-bit system is 56.06 dB.
manufacturer has the choice of developing complex and costly wideband,
brick-wall, ripple-free filters that yield an extended baseband frequency In retrospect
response, or a moderate cost, 4.2 MHz, low-pass filter with a gradual roll- D2/D3 digital composite VTRs appeared on the market at a time when
off. Betacam SP composite analog VTRs were in the process of capturing the
market. Betacam SP proved to be the more popular format, especially for
The sampling structure newsgathering, because of the availability of a field unit featuring a
The SMPTE 244M standard was developed with reference to the original piggyback camera, as well as complete compact editing systems. To avoid
(1953) specifications, which used I/Q encoding instead of the B-Y/R-Y multiple NTSC decoding/encoding picture-quality degradations, editing
encoding currently used. Figure 2 shows that any chrominance vector can suites used an S-Video connection with separate luminance and
be represented by I/Q or B-Y/R-Y vectors. The original intent of the NTSC chrominance paths. The performance figures of the D2/D3 VTRs were
standard was to assign different baseband bandwidths to the I signal (1.2 superior, especially if parallel or serial digital (143 Mbits/s) interfaces were
MHz) and the Q signal (600 kHz), thus providing a better resolution of the used. To this effect, several manufacturers offered digital composite
orange visual information. Figure 3 shows the block diagram of a typical production switchers with serial digital interfaces. In most cases, the D2/D3
1953 encoder. The first block of the encoder, the matrix, converts the VTRs were used as drop-ins in an NTSC analog composite environment.
gamma-corrected E´G, E´B and E´R primary signals into E´Y, E´I and E´Q The appearance of competitively priced component digital video equipment
signals. A low-pass filter limits the bandwidth of the E´I signal to 1.2 MHz has tilted the market in favor of component digital video.
and another low-pass filter limits the bandwidth of the E´Q signal to 600
kHz. The E´Y and E´I signals are suitably delayed to match the delayed
narrow-bandwidth E´Q signal. The two chrominance components feed
dedicated suppressed-carrier amplitude modulators in phase quadrature.
An additional subcarrier phase shift rotates the two vectors with respect to
the B-Y reference, as seen in Figure 2. A modern I/Q encoder as per
SMPTE 170M would use equal-bandwidth I/Q signals, so the low-pass
filters would be identical and there would be no need to delay the E´I
signal. The I/Q-encoded NTSC signal can be decoded along the I/Q axes,
with equal or unequal bandwidths, or along the B-Y/R-Y axis, with equal
wide or narrow bandwidths. In NTSC transmitters and receivers, the
baseband bandpass is limited to 4.2 MHz. Attempting to decode
chrominance signals beyond 600 kHz would result in severe I to Q or B-Y
to R-Y crosstalk due to upper chrominance vestigial-sideband effects, so
receivers never take advantage of wider-bandwidth chrominance
components when they are present. Very few I/Q decoding monitors or Composite digital video
Along period of concept, product and electronic component development as D2 (Sony and Ampex) and D3 (Panasonic) digital videotape recorders.
resulted in a large number of application-specific digital black boxes A wide range of compatible composite digital video studio-type production
equipment appeared on the market subsequently. The SMPTE 244M
standard defines the characteristics of the 4fSC NTSC composite digital
signals as well as the bit-parallel interconnect characteristics. The digital
signal aspects defined by the standard are summarized in Table 1.

The sampling structure

The sampling frequency is equal to four times the subcarrier frequency or
14.3181MHz (14.32MHz nominal). The sampling clock is derived from the
color burst of the analog signal. Figure 1 shows the sampling spectrum of
Table 1. Summary of coding parameters for 4fSC NTSC 4fSC NTSC.
composite digital signals Click here to see an enlarged diagram.
operating at incompatible sample rates, number of bits per sample and There is a significant gap between 4.2MHz (the maximum nominal NTSC
baseband frequency) and 7.16MHz (the Nyquist frequency). The standard
does not specify the characteristics of the anti-aliasing and reconstruction
filters. The manufacturer has the choice of developing complex and costly
wideband brick-wall ripple-free filters, resulting in an extended frequency
response, or moderate-cost 4.2MHz low-pass filters with a gradual roll-off.

As a result, various 4fSC products have different analog bandwidths. Note

that a digitally generated signal fed directly to a digital 4fSC unit will have an
equivalent analog bandwidth equal to fSC/2 = 7.16MHz. Severe overshoot
and ringing of the derived analog composite signal may result unless
special precautions are taken to ensure that digital blanking edges and rise
times, compatible with the analog waveforms, are included as an integral
Figure 1. Spectrum of a 4fSC-sampled NTSC signal. Click here part of the digital signal.
to see an enlarged diagram.
quantizing ranges. These products were developed to fulfill specific
production needs and were designed for analog composite video
interconnection compatible with the all-analog composite video production

The SMPTE 244M

standard was developed
with reference to the
original NTSC
specifications that used
I/Q encoding instead of
B-Y/R-Y encoding, as is
the current practice.
Figure 2 shows that any
chrominance vector can
be represented by I/Q or
B-Y/R-Y vectors. The
original intent of the
NTSC standard was to
assign different
Figure 2. Phase diagram showing the relationship between the bandwidths to the I
chrominance vector projections on the B-Y/R-Y axis system signal (1.2MHz) and to
and the I/Q axis system. Click here to see an enlarged diagram. the Q signal (0.6MHz),
studios. thus allowing for a better
resolution for the orange
The composite visual information.
digital video format
constitutes a The I/Q-encoded NTSC
stepping stone signal can be decoded
toward the all-digital
Figure 4. 4fSC NTSC sample numbering and along the I/Q axis, with
video teleproduction
horizontal sync relationship. Click here to see an equal or unequal
studio. In North bandwidths, or the B-
enlarged diagram.
America, there was Y/R-Y axis with equal
an initial interest in (equiband) bandwidths. Because the transmitter video frequency cutoff
composite digital occurs at 4.2MHz, the wider-bandwidth I signal is transmitted with unequal
videotape recorders. lower (-1.2MHz) and upper (+0.6MHz) sidebands (vestigial upper
This had to do with sideband), unlike the narrowband Q signal, which is transmitted with equal
the need to replace lower (-0.6MHz) and upper (+0.6MHz) sidebands. Few I/Q decoding
the obsolescent monitors and receivers were built because of decoding circuit
analog composite complications resulting in no visible picture improvements.
videotape recorders
with digital
videotape recorders
featuring analog
input/output ports.

A number of
manufacturers
developed such
products identified
Figure 3. 4fSC sampling instants of an NTSC
composite analog signal. Click here to see an
enlarged diagram.
 distribution, as detailed in SMPTE 259M, requires the reorganization of the
horizontal and vertical blanking intervals.

Figure 6 shows the location of the added five-word TRS (samples 790 to
794), as required by SMPTE 259M. This leaves space for 55 ancillary data
words (samples 795 to 849), which could be used for embedding four
digital audio channels.

The quantizing range

Figure 7 shows the relationship between analog NTSC signal levels and
eight-bit and 10-bit sample values of a 100/7.5/100/7.5 color bars signal.
The 10-bit approach provides for 1024 digital levels (210) expressed in
decimal numbers varying from 000 to 3FF. Digital levels 000, 001, 002 and
003 as well as 3FC, 3FD, 3FE and 3FF, are protected and not permitted in
Figure 5. 4fSC NTSC digital horizontal blanking interval
the digital stream. This leaves 1016 digital levels, expressed in decimal
showing the location of some significant samples. Click here to
numbers varying from four to 1019 or in hexadecimal numbers varying
see an enlarged diagram.
from 004 to 3FB, to represent the video signal.
As shown in Figure 3, the NTSC 4fSC standard requires that the sampling
instants coincide with peak positive and negative amplitudes of the I and Q The sync tip is assigned the value 16 decimal or 010 hexadecimal. The
subcarrier components. The upper part of the drawing shows that sampling highest signal level, corresponding to yellow and cyan, is assigned the
instants provide an adequate 4fSC representation of the B-Y/R-Y value of 972 decimal or 3CC hexadecimal. The standard provides for a
information. small amount of bottom headroom (some call it foot-room), levels four to
16 decimal or 004 to 010 hexadecimal, and top headroom, levels 972 to
Given a sampling frequency fS = 14.3181MHz (nominally 14.32MHz) and a 1019 decimal or 3CC to 3FB hexadecimal.
horizontal scanning frequency fH = 15734.25 Hz, the number of samples
per total line is equal to fS/fH = 910. The digital active line accommodates The total headroom is on the order of 1dB and allows for mis-adjusted or
768 samples (numbered 0 to 767). The remaining 142 samples (numbered drifting analog input signal levels. This reduces the S/QRMS (signal-to-RMS
768 to 909) comprise the digital horizontal blanking interval. quantizing error) ratio by the same amount. The theoretical S/QRMS of a 4fSC
product featuring analog in/out interfaces is 68.10dB for a 10-bit system
Figure 4 depicts the sample numbering for a nominal NTSC signal. The
and 56.06dB for an eight-bit system. This is considerably higher than any
half amplitude point of the leading (falling) edge of the analog horizontal
composite analog or component analog VTR.

Conclusion
In most cases, D2/D3 VTRs were used as drop-ins in an NTSC analog
composite environment. Their performance figures were superior to older
analog composite as well as analog component (BETA-CAM) VTRs,
especially if parallel or serial digital (143Mb/s) interfaces were used.

The major handicap of composite digital video was the fact that 4fSC could
not be compressed using highly efficient contemporary transform coding
methods typical of MPEG. Consequently, VTRs used a high recorded data
bit rate, 127Mb/s, resulting in large videocassettes and no portable
camera/VTR gear. The appearance on the market of competitively priced
component digital video equipment has tilted the market toward the
Figure 6. 4fSC NTSC horizontal sync period details showing adoption of component digital video equipment.
location of TRS-ID and optional ancillary data. Click here to see
an enlarged diagram.
sync signal falls between samples 784 and 785. The first of the 910
samples represents the first sample of the digital active line and is
designated sample 0 for the purpose of reference. The 910 samples per
line are, therefore, numbered 0 to 909.

Figure 5 details the digital horizontal blanking interval, showing the location
of some significant samples. Note that unlike component digital video,
where the horizontal digital blanking interval is not used — with the
exception of two four-word timing reference signals (TRS) — the 4fSC digital

Figure 7. Relationship between analog signal levels and digital

sample values. Click here to see an enlarged diagram.
signal carries horizontal sync and subcarrier burst signals as well. The
standard was designed with bit-parallel distribution in mind. Bit-serial signal
Component video Component video often refers to the three-cable attachments to consumer and
A video color format that maintains the three YUV video signals in three professional equipment such as DVD players, receivers, set-top boxes and TVs. Digital
separate channels. Component video provides a sharper image than component video (not shown here) uses only one cable.
composite video and S-video. See YUV, composite video and S-video.

Analog Component Video

Component video may refer to "analog" component video (YPbPr),
especially with regard to the Y, Pb and Pr cable connectors on devices
such as DVD players, set-top boxes, receivers and TVs. See YPbPr.

Digital Component Video

Component video may refer to "digital" component video (YCbCr), which is

the norm for tape formats such as MiniDV, DV and Digital Betacam. Digital
component video (YCbCr) is also natively supported by many nonlinear
video editing programs (NLEs). See YCbCr and YCbCr sampling.

RGB: Digital or Analog

Sometimes, component video refers to RGB signals rather than YUV. It

may refer to "digital" RGB, which is the native graphics format in the
computer, and it is supported by all nonlinear video editing programs
(NLEs).

Component video may also refer to "analog" RGB, especially with regard to
a three-cable RGB attachment to a studio monitor or high-end video
camera. See YUV.

Signal Comparison
The top diagram shows how YUV signals are mixed and distributed to outside connectors, and
the device on the bottom shows the actual ports from an NVIDIA display adapter. Note that the
red, green and blue sockets are not the red, green and blue of RGB. (Bottom image courtesy of
NVIDIA Corporation.)
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Analog Component Video (YPbPr)