Television is rapidly making the transition from standard definition analog TVs with CRT displays to high-definition digital TVs with LCD and plasma displays. While picture quality is greatly improved, several factors are making it difficult for TV manufacturers to improve or even maintain the audio quality that consumers are used to getting with CRT TVs:
- Customers want thinner TVs but the reduced depth of the cabinet forces manufacturers to use smaller speakers, hurting the low frequency audio response.
- To reduce the cabinet frame size, the speakers are sometime placed behind the screen. The sound is transmitted via a small acoustic horn, introducing tuned resonances.
- The thinner cabinets are also less rigid than CRT TV cabinets, introducing mechanical resonances.
- The wider aspect ratio of most flat panel TVs is also driving manufacturers to mount the speakers below the TV instead of the sides, reducing the stereo separation.
- The larger screen sizes of flat panel TVs means that the TVs are positioned further away from the viewers, also contributing to the reduction of the stereo separation.
At the same time, consumer expectations for TV audio are rising due to:
- The improved picture quality leads to higher expectations for audio. As customers are increasingly able to recreate the visual quality found in movie theatres, they also expect the same audio quality they are used to in theatres.
- DVDs and digital TV programming with multi-channel digital audio (e.g., Dolby, DTS etc.) provide higher-quality audio content.
Digital audio processing and amplification can be used to compensate for the problems created by flat-panel TVs and provide a high-quality audio experience.
Beyond compensating for problems, digital audio processing can also improve audio quality by creating the perception of center channel and rear speakers found in home audio systems, enhancing dialog clarity and improving sound quality at low volumes for nighttime listening, etc.
The Audio Path in Digital TVs
The audio for ATSC digital TV broadcasts is embedded with the video in an MPEG transport stream. The MPEG decoder in a digital TV decodes both video and audio (typically Dolby Digital) and provides a digital audio output (typically I2S) along with the video output.
In a simple configuration, the audio signal may just go through a volume control before being sent into an audio DAC and then out to an amplifier (figure 1). In actual practice some extra audio processing is added to improve the quality of the audio and synchronize it with the video (figure 2).
Figure 1. In a simple configuration, the audio signal may just go through a volume control before being sent into an audio DAC and then out to an amplifier.
TVs also need to be able to process both analog and digital audio during the transition from analog to digital TV. Analog TV signals use a subcarrier frequency for the audio, and analog tuners typically have a Sound IF (SIF) output that needs to be demodulated. DTV platforms typically have both an analog input for demodulating the SIF output from an analog tuner and a digital input from a decoder that handles both video and digital audio from the MPEG stream (figure 2).
Analog TV reception will probably be required past the analog broadcast cut-off date for compatibility with VCR, video games, and all audio/video peripherals that still output an RF-modulated signal.
Figure 2. In actual practice, extra audio processing is added to improve the quality of the audio and synchronize it with the video.
Lip Sync Delay
As video signal processing for display becomes more sophisticated, the video delay between the input and the display is getting increasing longer. This requires the TV to delay the audio to keep it synchronized with the video.
There are three ways to provide lip sync delay: 1) lip sync delay memory in the audio processor, 2) external lip sync delay memory attached to the audio processor, and 3) looping the audio back through the video processor in order to use the video processor's DRAM for the audio delay. Performing the lip sync delay in the audio processor is the easiest approach but it adds cost to the audio processor and lacks flexibility.
Using dedicated external memory for the lip sync delay adds more cost than integrated lip sync memory but it is easy and more flexible. Using the video processor DRAM is the least expensive approach but it increases the system complexity.
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