HDMI Fast Switching

Background: What is Fast Switching?

User experience is the key to any consumer products’ success. The technical superiority and the functionality of the product must be backed by ease of use and exceptional user experience. If the user experience is not optimal, consumers will find something better, and they’ll find it quickly.

Take, for example, the process of switching between the HDMI inputs on a television. Switching between multiple HDMI inputs can take up to ten seconds—a noticeable, and irritating, delay. During this ten second period, the user sees a blank blue screen which reads: “? Searching for HDMI signal”. The consumer experience in this function has much room for improvement. In many cases, the consumer gets impatient and gets the impression that the DTV is ignoring the request, and continues with the futile pressing of the Input button.

Technology is now available that can significantly reduce or remove this consumer frustration. Fast switching technology integrated into the Synopsys DesignWare HDMI Receiver IP Solution improves the user experience considerably. They will notice a much faster response time in the HDMI receiver (i.e., DTV) as they switch between the multiple HDMI inputs that are connected to the DTV. For example, the DTV HDMI inputs can be connected to digital video recorder (DVR), multimedia box, game console or even a camcorder. The fast switching technology built within the DesignWare HDMI Receiver reduces switching time between these devices by up to 90% – from ten seconds to about one second.

How Fast Switching Works

To understand how fast switching works, let’s first define two simple terms: “connection” and “selection”. “Connection” is the process of connecting the HDMI wire from a source device (such as a DVR) to a sink device (such as a DTV). “Selection” is the process of pressing the input button of the remote control of the sink device (DTV) to choose the desired HDMI input. Figure 1 shows an example of a DTV connected to a set-top box and a Blu-ray player. In Figure 1A, the set-top box is the active HDMI input on the DTV. Figures 1B and 1C show the process of switching the active input from the set-top box to the Blu-ray player, with and without the fast switching technology.

In Figure 1B, without the fast switching technology, switching from the set-top box to the Blu-ray player can take up to ten seconds. During this interval, the user will see a blank blue screen. Figure 2 shows that a system without fast switching waits until the user selects the input on the remote control to switch from the set-top box to the Blu-ray player. The hot plug detect process initiated only upon user selection. The Blu-ray player then reads the DTV’s extended display identification data (EDID). Once the EDID content is read, the DTV begins the process of high-bandwidth digital content protection (HDCP) authentication with the Blu-ray player. This whole process can take up to ten seconds.

However, with fast switching, when the set-top box and Blu-ray player are initially connected to the DTV, the hot-plug detect signals of both HDMI inputs are recognized and the receiver terminations are activated on both inputs. The DTV provides the EDID information to both the set-top box and the Blu-ray player, and the HDCP authentication process is initiated for both. However, the DTV only authenticates HDCP for the selected device, which in Figure 1A is the set-top box. For the other, non-selected, device (Blu-ray player), the receiver will not acknowledge the TX HDCP bus and will fail the link authentication and link integrity check in the very last step of the HDCP authentication process. However, the non-selected input (Blu-ray player) has already completed most of the handshaking process, including obtaining EDID information from the DTV, and a large proportion of the HDCP authentication (such as determining DVI vs. HDMI, “content protection desired cycle” etc.) The non-selected input will now stay in the A0 state of the HDCP authentication process and will wait for the user to select it prior to completing the final A0 state. Then, when the user selects the Blu-ray player to be the active input on the DTV remote control, the user only has to wait for the final state of the HDCP authentication process to complete—which only takes about one second.

While most commercially available transmitters in the market will send the re-authentication request in less than one second, the typical switching time can be up to two seconds due to the HDCP specification. It states, “Should the authentication fail, the HDCP Transmitter must retry periodically, with a period of no more than 2 seconds.” Hence, the actual switching time is limited by the period between the re-authentication requests from the transmitter, which is defined to be a maximum of two seconds.

Power Considerations

As HDMI is a DC coupled interface, it uses 3.3V terminations on the RX side. When fast switching is turned on, the HDMI PHY turns on RX termination on all four channels. There are four lanes (three data lanes – red, green, blue, and one clock lane) in each of the four channels. By the definition of the HDMI specification, there is a ~12mA current in each of the four lanes of each of the four channels. This leads to additional power consumption as only one of the four channels is connected and selected, but the other three channels have to be connected with DC terminations enabled for fast switching between the four channels. The total power consumption required to enable fast switching is approximately 12mA per lane, or 48mA per channel. When all four channels are turned on, the total power in the terminations is 48mA per channel x 4 channels x 3.3V = ~ 630mW. To better manage power (cut down the termination power by 75%), the fast switching feature can be disabled in the software, which cuts the power consumption in the three unselected channels.

DesignWare HDMI Receiver IP with Fast Switching

The Synopsys DesignWare HDMI Receiver IP leverages a complex software stack and a combination of digital logic controller and advanced mixed signal techniques in the PHY to implement the fast switching solution. The HDMI RX PHY provides up to four input channels to implement the fast switching logic. As shown in Figure 3, the handshaking mechanism for fast switching is carried out in the SCL, SDA, and HPD (pins 15, 16, and 19) signals of the connector. It does not involve the main TMDS signals at all. Once the HPD is sensed from all four connected TX channels, the HDMI RX PHY will turn on the terminations on all four input channels. The SCL and SDA lines will then carry out the exchange of EDID information and HDCP authentication process between the HDMI receiver (DTV) and the four HDMI transmitters. The HDMI RX software stack manages the exchange of EDID information with all four channels in parallel and process of authenticating one channel and keeping the other three channels on hold in the final stage of the HDCP authentication process.

As the users of home theater systems become more demanding, fast switching features will go a long way in differentiating products and improving the overall user experience. To learn more about Fast Switching Technology, view the Fast Switching demo video, or visit the DesignWare HDMI web page.