The error correction and error masking in modern digital equipment ensures that digital signals do not gradually degrade with increasing attenuation in the signal path as analog signals do. Instead, a digital transmission path continues to work perfectly up to the point where it suddenly does not work at all — the well-known cliff effect. As signals approach the digital cliff, errors go rapidly from nonexistent to severe — swamping recovery efforts and making the path unusable.
Bandwidth and signal requirements
Although the SDI signal is digital in nature, it has many analog qualities that can be used to predict how close to the error cliff a particular digital path is. The SMPTE292 bit-scrambling algorithm essentially produces a square-wavelike signal. The peak-to-peak value of this signal should be 0.8V and the rise time or transition time between the 20 percent and 80 percent amplitude points should be only 270ps. Using the distance of New York to Los Angeles to represent one second in time, the total time taken up by a single bit cell would represent less than half a centimeter travel. The HD bitstream has a fundamental frequency of 750MHz. If the third harmonic is added (3 × 750MHz), the resulting waveform resembles a square wave. If additional odd harmonics were added in the correct amplitude and phase, a nearly perfect waveform would result. In reality, with the extremely high bit rates needed for HD, only the third harmonic makes it more than a few feet down a coax.
Figure 1. This graph shows the energy loss an HD signal suffers as it travels in a coaxial cable. After traveling about 250 feet, fundamental energy drops to a third of its original value, while the third harmonic drops to the noise floor, making the HD signal unrecoverable. Click here to see an enlarged diagram.
The physical layer used to transport the data is composed mostly of coax, with some connectors and perhaps a jackfield. But coax provides the greatest exposure to problems for a video datastream. Coax's series inductance and shunt capacitance create a low-pass filter. Because attenuation is greater in the higher frequencies, the upper harmonics of the signal disappear. The square wave data signal starts to look more like a sine wave and may become unrecoverable.
The bottom line
The weight of the cabling needed in a broadcast facility often equals the weight of the equipment. For this reason, facilities, especially trucks, are built with the lightest cables possible. Many facilities use minicoax, even with HD. Unfortunately, smaller-diameter cable results in increased HF losses. RG-59 type coax might show 5dB loss per 100 feet at 750MHz, whereas minicoax has 9.59dB of loss. At 2250MHz (the center of the HD third harmonic) RG-59 type typically has 9.14dB loss per 100 feet, while minicoax has a loss of 16dB.
Figure 1 graphs energy loss against distance for a coax path. It shows that third harmonic energy drops at a much greater rate than the fundamental energy as distance increases. This is why loss at both levels must be considered.
Usually the error headroom value is checked with expensive test equipment, such as a spectrum analyzer. 4sight's handheld HRM-1500 meter offers broadcasters designing, building and maintaining HD facilities a simple, economical way to check the health of HD signal paths. It determines the amount of energy in the third harmonic, which indicates how close to the error cliff the HD bitstream is. It also provides a simple indication of distance from the error cliff, more detailed information on overall bitstream energy, and a breakdown of fundamental and third-harmonic energy.
Jim Boston, Andy Hutton, Lou Janis and Rob Martin are engineers with 4sight. For more information on the HRM-1500, visit www.4sightproducts.com.