I enjoyed your explanation of digitizing audio in the May 2006 issue of Broadcast Engineering. However, I have a couple questions: Where does the 44.1kHz sampling rate come from? Why not, for example, 42kHz or 44kHz?
Michael Robin responds:
Digital audio bit rate is about 1Mb/s per audio channel. Early digital audio equipment had difficulties storing these signals. To solve the problem, video recorders, such as Sony U-matic, were adapted to store audio samples by creating a pseudo video waveform, which conveyed binary as white and black.
The sampling frequency of such a system requires a simple relationship to the field frequency and field structure of the related TV standard. The result is an integer number of samples to be stored on each usable TV line in the field. The allowable audio sampling frequencies in a pseudo video system are obtained by multiplying the field frequency by the number of active lines in a field and by the number samples in a line.
In the 525/59.94 video, assuming 490 active lines or 245 lines per field and storing three samples per line, the sampling frequency is 59.94 × 245 × 3 = 44.0559kHz. In the 625/50 video, assuming 588 active lines or 294 lines per field and storing three samples per line, the sampling frequency is 50 × 294 × 3 = 44.1kHz. There is a 0.1 percent difference between these two frequencies. For some purposes, recordings made at one rate can be played back at the other. The equipment used to make CD masters is video-based and thus determines the sampling frequency.
Essentially, the VCRs are run at 525/60 — not at 525/59.94 — and the resulting sampling frequency is 60 × 245 × 3 = 44.1kHz. This is how 44.1kHz became the sampling frequency of a compact disc.
Can you provide information on gamma correction for a PAL system?
John Luff responds:
NTSC and PAL analog composite television form the base of much legacy technology. NTSC is specified in SMPTE 170-1999, and PAL is specified in ITU-R BT.470. The precise details of BT.470 can be purchased online at www.itu.org for 25 CHF (about $25). SMPTE standards are available at www.smpte.org.
Gamma was enshrined in our standards to account for the nonlinear response of a CRT. With many new display technologies, the transfer characteristics are different; however, we need to continue to provide pictures with gamma applied in order to be compatible with legacy displays. Every new display must account for this fact with internal processing prior to display.
NTSC is specifed with a gamma of 2.2, while PAL is specified with a gamma of 2.8. However, according to many experts, including Charles Poynton (see Poynton's “Technical Introduction to Digital Video,” published by John Wiley & Sons), 2.8 is unrealistically high. ITU-R BT.709 specifies the colorimetry in use in much of the television industry, including HDTV systems, and uses a somewhat more complex transfer curve with a short linear segment near black. BT.709 specifies a power function with an exponent of 0.45, which is about equivalent to a gamma of 2.222. In practice, both NTSC and PAL today are more likely to be encoded using Rec. 709 colorimetry and transfer functions than the original specifications of decades ago.
Sometimes it's hard to know all of the new broadcast abbreviations and terminology. Does Broadcast Engineering have an updated glossary of DTV?
Alfredo Flores Félix
Sureste, S.A. de C.V.
Villahermosa, Tabasco, México
Editorial Director Brad Dick responds:
Broadcast Engineering does not maintain a database of acronyms. However, here are a few Web sites that provide definitions to unfamiliar terms: http://acronyms.thefreedictionary.com, www.acronymfinder.com and http://isp.webopedia.com.
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