Wireless Home Digital Interface (WHDI) Explained

The promise of Wireless Home Digital Interface, or WHDI, is to eliminate all of the in between steps for connecting a home entertainment system except for the power supply. According to their site, "WHDI is the new wireless High Definition video standard that will change the way people use Audio/Video devices in the home. WHDI's revolutionary video-modem approach enables top-quality and robust wireless uncompressed HD video delivery throughout the home, allowing consumers to connect any source in the home to any display."

The WHDI Special Interest Group, created in July 2008, is made up of a number of consumer companies, including LG, Motorola, Hitachi, Sharp, Sony and Samsung, as well as component manufacturer Amimon. They are still a few months away from finalizing the standard, but Amimon has already announced their next generation baseband chipset. While the chipset is available now, product implementing the solution will likely start to appear at the end of the year, with mass traction expected in the second half of 2010.

WHDI works with a 20MHz or 40MHz space on the unlicensed 5GHz band to wirelessly send signals from a component to a display. Due to the bandwidth used, the signal has a 100 foot range and will go throughout the home, including through walls, so there are no physical barriers for the system setup.

The signal is uncompressed data. In the past, companies that have implemented a wireless HDMI signal compressed the data, transmitted it, and uncompressed it at the display. This increased the latency of the signal as it takes time to compress and uncompress the information, meaning that it is not an ideal solution for playing gaming systems. With WHDI the signal is uncompressed, meaning that the output is directly sent to the display, allowing for a latency of approximately 1ms, and is format agnostic so any output can be transmitted.

The WHDI standard offers a number of improvements over the original solution that was used previously. It can support 1080p signals, instead of only 1080i, and supports DSS and higher graphic resolution. Previously, a manufacturer had to implement both the transmit and receive portions of the link. When the standard is released then there will be a common platform so that a manufacturer only needs to supply one side of the path and can rely on others for the correct signal interpretation.

A typical system would have the WHDI receiver built into the TV so that it is available upon purchase. A converter box would accept wired signals from various sources, such as a set top box, PC, or gaming system, and transmit the signal to the TV. If the TV does not have the receiver a receiver box can be added to enable to functionality.

To ensure that the signal is clean and clear, the standard implements a frequency agility technique, which is a combination of frequency and spectrum control. When the link is initiated it looks for a free channel so that there is no interference, and continually monitors the channel to determine if something is encroaching on the bandwidth. If there is interference the WHDI signal will switch to another channel, thus allowing it to co-exist with other wireless product in the home.

To make the technology a reality the WHDI created a new type of transmission based on a video modem as opposed to a data modem. In a traditional data modem, which most wireless HD products are based, every bit is treated with equal importance. So while the error rate might be low, there is a large amount of data being sent that must all be accurate to generate a quality image. In a video modem the signal is broken into thousands of different layers. There is an algorithm to determine which layers are more important, and these have a higher level of protection. The less important layers have less protection so they are more error prone, but as they have little impact on the picture quality so it is not noticeable.

A way to think of this is to consider each layer as an 8-bit code, giving 256 different choices. The most significant bit is protected because that can make quite a large different in the outcome, where as the least significant bit will have much less impact. The LSB is still sent as it is important, but if the error of that bit is larger it is not as prevalent in a video image.

The exception this method is audio. With an uncompressed signal being sent if the user is sending a considerable amount of audio signals through a WHDI link this can affect the image quality as there is only so much bandwidth to be used. However, the majority of the people that are going to encounter this issue, which will likely not be that many, are likely going to have independent wiring for their sound system to ensure the best quality possible.