TV stations usually concentrate on transmitting HDTV content to distant locations using as little data bandwidth as possible while maintaining quality. Amimon’s new WHDI chip set transmits HDTV to nearby devices using as much data bandwidth as possible.
By using Joint Source-Channel Coding (JSCC) instead of conventional data transmission methods that treat each bit as equally important, Amimon’s WHDI system is able to transmit data rates between 250 and 800 Mbps using a 20 MHz wide 5 GHz channel over short distances inside a house. 8-bit per color HDTV samples can be compressed to about half their original data rate, 1.5 Gbps for 720p and 1080i HDTV or 3.0 Gbps for 1080p HDTV. On a good 20 MHz wide channel (or two for 1080p), Amimon’s WHDI system is able to transmit a near perfect picture.
What happens when the channel is degraded and the data rate drops below half the uncompressed rate? That’s where JSCC makes the difference. Video components are prioritized according to their importance. For example, most significant bits are more important than least significant bits, lower spatial frequencies are more important than higher frequencies and the luminance component is more important than the chrominance components. Higher priority components receive more error correction than less important components. High priority components are also transmitted using a less complex modulation method—a coarser constellation, for example, transmitted on frequencies with less noise. As the channel SNR deteriorates, lower priority components are lost first.
Compare WHDI to broadcast DTV transmission methods, which heavily compress the video using complex and expensive encoders before transmitting the data over a system that requires considerable error correction and has little tolerance for loss of data—the “all or nothing” cliff effect. WHDI is not a simple WiFi type system. Amimon uses MIMO (multiple-input, multiple-out) technology with four antennas to allow “space-time” modulation, enabling higher data rates and more robust transmission.
Amimon’s Web site has an excellent technical paper describing in more detail, including an interesting analysis of the data capacity of a channel.
By now, you may be wondering if WHDI could be applied to video systems working over distances measured in miles rather than tens or hundreds of feet. Probably not, as 20 MHz of licensed bandwidth for long-range communication isn’t easy to obtain in congested urban markets, especially for a MIMO system. However, a variation on this system, using less complex and more robust modulation for MPEG “I” frames versus the “B” or “P” frames carrying only incremental picture changes, might provide a small improvement in system performance. I say small because the amount of data in the B and P MPEG frames is usually small compared with the amount of data in an I frame. Because MPEG compression systems throw away most of the visual frame-to-frame “redundancy”, each bit is more important.
One lesson from this is that technology that may once have been considered impractical, such as prioritizing HDTV video components and using the result to hierarchically modulate a MIMO transmitter system in what is essentially real time (1 millisecond) is now practical for consumer equipment, thanks to advances in chip manufacturing and design.
Comments and story leads are always welcome. Email me at dlung@transmitter.com.
