HD and SD scopes

The transition from analog to digital production and transmission technology impacts the tools of our trade in fundamental ways. But manufacturers have done a great job of camouflaging some of these changes. This smoke-and-mirrors act might seem strange at first, but it has a very positive purpose: to ease the transition. From the general-purpose oscilloscope, manufacturers have adapted a specialized version, the “waveform monitor,” to simplify its use for video and TV — and now, for digital TV. (See Table 1.)

Table 1. The waveform monitor was adapted from the general-purpose oscilloscope for use as a specialized broadcast tool by restricting it to a vertical calibration of roughly a volt top to bottom and roughly 64 µs.

NTSC can be encoded from any combination of legal RGB values, but only as long as the encoded components remain within the gamut of SMPTE 170M-1999. In legal NTSC colors, the maximum excursion of the subcarrier must fall between -23 and +120 IRE, which is the cutoff of the carrier. Legal RGB colors can, however, create 131IRE excursions. Therefore, it is important to ensure that the component signals cannot create “illegal” encoded signals (even though they are perfectly legal component video). Standard-definition scopes are normally equipped with special modes that easily display such errors, either by an error indicator or a graphical display such as the Tektronix “Arrowhead” display. This specialized display shows gamut errors for composite, RGB or component signals. As a convenience to operators, several manufacturers create a pseudo-composite display by locally encoding the component waveform into an NTSC or PAL signal. This can be especially valuable in easing the transition to a digital system.

Most modern scopes allow both waveform and vector modes, and quite often display other information about the signal. For SD or HD video, it is valuable to know if there are errors in the signal, whether reported by EDH flags or detected by the embedded CRCs in the digital signal (full-field and active-picture CRCs are available). Data screens can also display important information such as the presence of embedded audio, the absence of SAV or EAV data, or scope reference errors. In addition, the analog SMPTE 259 or 292 waveform and the “eye” pattern created by the embedded clock and data often show the health of the digital carrier. The scope may also display jitter waveforms or histograms to provide insight into the overall health of the signal.

Sophisticated combined displays can be created by digitizing the signal and displaying the results on a computer monitor. Photo courtesy Videotek.

In the last decade, waveform displays have evolved to include a class of devices based on digitizing the signal and displaying the results on a computer (or analog video) monitor. These very sophisticated combined displays can show the waveform, vector display, audio signals (from embedded or discrete input sources), and the picture itself subsampled to one-quarter screen. Such monitors also allow the user to bring any of these displays to full screen and show them on large monitors for easy viewing. They include overlaid graticule information that can be very accurate, since the display requires no calibration. Many facilities put such a display on a router bus output and then re-enter the composite signal into the router so the waveform monitor output can be brought up on QC monitors at many locations without buying as many devices. The fact that these displays do not have their own CRTs can offer a long-term savings in maintenance cost. Such a device also does not need to be calibrated as often, though all analog input devices must be calibrated occasionally. Digital display of digital waveforms is inherently accurate and should be more stable.

HD waveform monitors are little different in the component digital domain, except for the multiplicity of standards that they often must accommodate. The predominant picture formats in North America (1920×1080i and 1280×720p) have been joined by 24-frame progressive variants, as well as 1080psf (progressive segmented frame). All of these, as well as the 60/59.94 variants, are in common use, along with 50-, 25-, 24- and 23.98- (24/1.001 for 525/59.94 compatibility) fps, and all can be mapped into SMPTE 292 at 1.485Gb/s. You may well be able to use a scope with fewer optional formats but, considering the somewhat fuzzy future of our industry, keeping options open at this point in the DTV transition is a wise move. Scopes for HD and SD use may well have to accommodate a total of at least 28 distinct formats, with 625 and 525 variants being only a small subset.

Standard-definition scopes often come equipped with error indicators or a graphical display like the Tektronix “Arrowhead” gamut display to en- sure that legal component video signals do not create illegal signals when encoded.

Manufacturers have devised a dizzying array of options. The best way to choose the right scope for your new digital plant is to have a couple of manufacturers bring in their product and demonstrate the operating modes, as well as show how it will integrate into your facility. Unfortunately, in making your decision, you will also likely have a thick book to digest. You may decide a handheld unit is all you need, or choose a combination HD/SD unit with separate VGA output for a large screen. All are readily available.