By Mika Iisakkila
Comments and corrections are welcome to me by e-mail. My email address is [email protected]
Notes on specifications and nomenclature
Analogue recording formats
Uncompressed digital formats
Digital formats employing compression
D series formats
DVC / DVCam / DVCPRO
WHAT THIS DOCUMENT COVERS AND WHAT IT DOESN’T
There seems to be a lot of confusion about the huge number of different video recording formats and their specifications. Information is widely scattered and hard to find. This document will mostly cover professional and semi-pro formats, both those in use now and those being under development.
For the time being, there isn’t much information on analogue formats, but I will be adding more stuff as I come across it. Unless noted, all information applies to PAL 625/50 versions. I’m omitting SECAM on purpose, because it’s rarely used for studio work anyway.
Many of the figures have been extracted from manufacturers’ brochures and do not necessarily represent the best a given recording format can offer. Nevertheless, they should be pretty close.
And to keep things in perspective, let me remind you that 16 mm film beats the living daylights out of most formats depicted here, especially when it comes to dynamic range.
Notes on specifications and nomenclature
BROADCAST QUALITY DEFINED – NOT!
The rest of this document, as well as many other texts handling
video production, will frequently refer to “broadcast quality” without really defining what it is. Such a definition doesn’t really exist — many people think of it as describing the minimum quality for a program that is to be broadcast. There is no technical standard for this, and in fact,almost any crap can be restored to good enough quality with time base correctors and other digital processing.
From the technical point of view, broadcast quality can be thought of as a video signal fulfilling the timing and signal level tolerances placed by the relevant international standards. This doesn’t say anything about the bandwidth (resolution) or signal-to-noise ratio of the actual picture — that remains to be judged from the highly untechnical point of view: considering the contents of the program, is the picture watchable enough? Now that there are plenty of good recording formats available, broadcast quality recording usually means something that isn’t noticeably worse than direct composite video from a good camera.
COMPOSITE VS. COMPONENT
In this context, component video means colour video represented by three separate signals (luminance and two colour difference signals). This is commonly referred to as Y/Cr/Cb colour space. RGB is almost never used in recording, because it requires 3/2 times the bandwidth of Y/Cr/Cb representation in order to achieve similar subjective quality.
Some video equipment may still have RGB inputs or outputs, as converting between these two component representations is easy.
S-Video (or Y/C) does not count as component video, as most externally composite formats use separate chrominance and luminance signals in recording anyway. Additionally, the chrominance signal of S-video is already modulated in either PAL or NTSC to ride on a subcarrier and as such is limited in bandwidth. Component video does not have this limitation.
BANDWIDTH AND LINES
Both of these figures refer to the horizontal resolution. “Lines” refers to the number of vertical alternating black and white lines that can be stuffed in the picture and still be perceived as separate lines and not a gray mass. It’s clear that this is not a very scientific definition of resolution when the signal is in the continuos domain, like it is with all analogue formats. Therefore, the bandwidth is better expressed as the real electrical signal bandwidth fitting inside some dB limits, which can be easily measured.
As you are bound to come across bandwidth specifications in both lines and frequencies, here’s how to convert from lines to bandwidth:
4/3 * resolution in lines
BW = —————————– / 2.
length of active picture line
The divisor 2 comes from the fact that you need two lines (black and white) to represent a single sine wave cycle. To make things complicated, even the horizontal line resolution is usually expressed as the number of lines that could be reproduced vertically, if the horizontal and vertical resolutions were equal. Therefore the equation contains the multiplier 4/3, which comes from the aspect ratio of television screen. I doubt that all advertisers and salesmen know this.
For 625/50 PAL, the length of the active (visible) part of the picture line is 52 us, so for example the 240 line horizontal luminance resolution of VHS becomes
4/3 * 240
——— / 2 = 3 MHz
worth of bandwidth. Of course, we don’t know the dB limits for
this figure, because nobody told us how gray those 240 black and white lines got. For this reason, respectable +/- 1 dB bandwidth specifications may look worse than the same picture quality expressed in lines (which is why lines are preferred in advertisements). Reversing the equation to get from bandwidth to lines is left as an exercise to the reader.
In the specs for component formats, the expressed colour bandwidth applies to both of the colour components separately.
Note: This simplified discussion omits some important points, but will suffice for the usual 4:3 aspect ratio TV systems. If you want the dirty details, consult a good text book like . Furthermore, this conversion method isn’t valid for chrominance bandwidth and resolution (I’m still trying to figure out exactly why, but I suppose it’s because the colour bandwidth in composite video is limited to some 1.5 MHz in any case).
BUT WHAT ABOUT VERTICAL RESOLUTION?
Unlike the horizontal direction, the vertical direction of television picture is discrete and not continuous. Still, on the picture tube the lines overlap and are not normally perceived as separate. For this reason (and others, like the Kell factor, but I’ve digressed enough already from the original point of this document), the vertical resolution of TV is not such a big deal. If it were, people would have abandoned 525 line NTSC long time ago. As all of the analogue formats record picture lines totally independently of each other (with some exceptions, see below), there is no need to state the vertical resolution. It is always the same as in the video format itself — 575 visible lines in PAL, 485 in NTSC.
The same story applies to the digital formats which are represented here. None of them throw away every other line, like CD-i for example does. Even the formats using compression maintain the entire vertical resolution, although all the compression methods are two-dimensional and operate on the image as a whole and not on separate lines.
One can argue that that in PAL, the principle used for colour phase error cancellation reduces the vertical chroma resolution by half. But, this doesn’t happen until in the receiver – the video recorders and the PAL video itself still carry the full vertical colour resolution. The exceptions here are VHS and S-VHS, where the method used for track-to-track chrominance crosstalk reduction really cuts the vertical chroma resolution to half on the tape.
The chrominance also moves down in the picture.
This is hardly notable in the first generation (cf. PAL), but
unfortunately degenerates during each copy, so second and third generation VHS copies get barely acceptable and poor,
respectively. The results can be seen as those famous
Analogue recording formats
M-II EBU-C Betacam SP U-MaticSP S-VHS
Component Composite Component Composite Composite
5.0 MHz 5.5 MHz 4.5 MHz ~ 4 MHz ~ 5 MHz
1.8 MHz 1.5 MHz 1.5 MHz ~ 600 kHz
> 47 dB 43 dB > 51 dB > 46 dB 45 dB
> 50 dB 43 dB > 53 dB > 48 dB
2 x FM, 3 x linear 2 x linear 2 xlinear,
2 x linear (2 + time code) 2 x AFM, TC time code S/N
> 85 dB > 72 dB > 52 dB > 90 dB
1/2″ cass. 1″ open reel 1/2″ cass 3/4″ cass 1/2″ cass
6.6 cm/s 24.4 cm/s 10.15 cm/s 9.53 cm/s 2.399 cm/s
Uncompressed digital recording formats
D-1 D-2 D-3 D-5 D-5 D-6
Component Composite Composite Component Component Component,
4 Fsc = Y: 13.5M Y: 18M
17.7 MHz C: 6.75M C:6.75M
4:2:2, 8 bits 8 bits CCIR601, 4:2:2,
8 bits 10 bits 8 bits
Luma 6.0 MHz Y: 5.75M Y:7.67M
Chroma C: 2.75M C:3.67M
54 dB > 60 dB > 56dB
Data rate 270 Mbps
4 x 48k, 4x48k @ 4x48k @
digital 16 bits 20 bits +
3/4″ 3/4″ 1/2″, 1/2″,
DIGITAL FORMATS EMPLOYING COMPRESSION
I haven’t been able to find any figures about bandwidth or resolution forÂ some of these formats (many are still under development). One can presume that most of these use quantization according to CCIR Rec. 601, , which is widely considered as the specification for “broadcast quality” when it comes to digital video. The most common CCIR 601 subformat is 4:2:2 quantization with 720 active samples per line. 4:2:2 means Y/Cr/Cb (YUV) format video, where the sampling rate for both of the colour difference signals is half the sampling frequency of the luminance.
Digital Digital S Ampex DCT DVC/ Betacam SX
4:2:2, 4:2:2 4:2:2, 4:2:0 / 4:2:2
8/10 bit 8 bit 8 bit 4:1:1(seebelow)
per field intraframe DCT, 2:1 5:1 -> 25 Mbps, 10:1 MPEG-2,
DCT, 2.3:1 DCT 3.3:1, per field/frame main level
S/N 54 dB
4 x 20bit 4 x 16 bit 2 x 16 bit/ 4 x 16 bit
48kHz PCM 4 x 12 bit PCM 48 kHz
Copyright Â© 1995-99 Mika Iisakkila. Reproduction of this document or any parts of it in any other form than linking to it via WWW is forbidden without permission from the author.