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Televisions are rapidly shifting from standard-definition analog TVs with CRT displays to high-definition digital TVs with LCDs and plasma displays. However, despite the dramatic improvement in picture quality, there are still many factors that make it difficult for new TVs to obtain or improve the audio quality of CRT TVs.
• Users want to use thinner TVs, but shrinking chassis thickness forces manufacturers to use smaller speakers, reducing audio response at low frequencies.
In order to reduce the size of the TV frame, the speaker is sometimes placed behind the screen, and the sound is sent out through a small acoustic horn, which easily causes tuning resonance.
• The thin case of a flat-panel TV is more likely to cause mechanical resonance than a CRT TV case.
• Most flat-panel TVs are getting wider and wider, forcing manufacturers to mount speakers under the TV instead of on both sides, affecting stereo separation.
The larger the screen size of a flat-panel TV, the more distant the TV is from the viewer, which also results in a reduction in stereo separation.
On the other hand, consumers' expectations for TV sound effects are getting higher and higher, mainly due to:
• Improved picture quality increases the expectation of audio quality. As more and more users are able to reproduce the visual effects in the cinema, they also want to feel the same audio quality as in the cinema.
• DVD and digital TV programs with multi-channel digital audio (such as Dolby, DTS, etc.) can provide higher quality audio content. Digital audio processing and amplification can be used to compensate for these problems with flat-panel TVs and provide a high-quality audio experience.
In addition to compensating for these problems, digital audio processing can also improve audio quality, enhance dialogue clarity, and enhance late-night viewing of TV programs at low volume by recreating the central channel and post-fader sensations found in home theater systems. The quality of the sound.
Audio channel in digital TV
The audio and video of the ATSC digital television broadcast are embedded in the MPEG transport stream. The MPEG decoder in a digital television simultaneously decodes video and audio (usually Dolby Digital) and provides digital audio output (typically I 2 S) and video output.
In a simple configuration, the audio signal may be sent to the audio DAC only after volume control and then output to the amplifier (Figure 1). In the actual application, some additional audio processing circuits are added to improve the audio quality and synchronize it with the video (Figure 2).
Figure 1: In a simple configuration, the audio signal may be sent to the audio DAC only after volume control and then output to the amplifier.
Figure 2: Add some additional audio processing circuitry to improve the audio quality and synchronize it with the video.
In the transition from analog to digital television, televisions also need to be able to handle both analog and digital audio. The audio in the analog TV signal uses a subcarrier frequency, while the analog tuner usually has a sound intermediate frequency (SIF) output that needs to be demodulated. The DTV platform typically has an analog input for demodulating the SIF output from the analog tuner and a digital input from the decoder, and the latter decoder is used to process the video and digital audio in the MPEG data stream (Figure 2).
The analog TV receiver circuit may be needed even after the analog broadcast is turned off, because it is compatible with VCRs, video games, and all audio/video peripherals that still output RF-modulated signals.
Lip Sync Delay
As the processing of video signals for display becomes more complex, the video delay between the input and the display is also getting longer. This requires the TV to delay the audio signal to keep it in sync with the video signal.
There are three ways to achieve lip sync delay: 1) using the lip sync delay memory in the audio processor, 2) using the external lip sync delay memory connected to the audio processor, 3) looping the audio back The video processor uses the DRAM of the video device to implement audio delay. Implementing lip sync delays in audio processors is the easiest way, but it adds to the cost of the audio processor and lacks flexibility.
Using a dedicated external memory to achieve lip-sync delays increases the cost more than integrated lip-sync memories, but it is also simpler and more flexible. DRAM using a video processor is the least expensive method, but it adds to the complexity of the system.
Graphic equalization
The first step in frequency response correction is to illustrate the equalization. The graphic equalization uses a bandpass filter to boost or lower the gain of a particular band. This is a great way to give users a good control over the frequency response because they are very familiar with the interface for this type of processing.
Graphical equalization can be used to compensate for audio problems caused by small speakers, chassis resonance, etc., but it is not the best method because the width and frequency of the band are fixed.
Figure 3: LCD TV audio system with parametric equalization for better frequency response.
Parameter equalization
The parametric filter can also be used to adjust the response curve of the TV amplifier. The parametric filter is better than the graphical equalizer in correcting poor frequency response and resonance problems due to small speakers and ultra-thin chassis, because the parametric filter can be programmed to adjust the overall filter response, center frequency, and Q.
Biquad filters are often used for parametric equalization. The digital biquad filter also has great flexibility because it has two poles and two zeros that can be used to adjust the filter response to form a low pass, high pass or band pass or even a notch. The pole and zero positions in the biquad filter can be changed by adjusting the filter coefficient values.
Improve stereo separation
As digital TV sets increase in aspect ratio, stereo separation should be larger and larger, but more and more TV manufacturers put speakers under the display instead of on both sides, in order to reduce TV The overall width of the machine. Placing the speaker under the display will reduce the stereo separation and will also move the sound center from the center of the display to the underside of the display.
Algorithms from companies such as SRS Labs, BBE Sound, and QSound Labs can be used to increase the size of the audio image, improve the perceived bass performance, and produce the effects of the center and rear channel speakers on both speakers. These algorithms can improve the perceived audio quality of a television set and produce a listening system similar to a multi-channel home theater system. Figure 4 shows how BBE ViVA can improve audio panning (BBE Sound).
Figure 4: How BBE ViVA can improve audio and video (BBE Sound)
Dynamic range optimization
It is difficult for small TV speakers to reproduce the full dynamic range contained in the digital audio track. Expanding the dynamic range is also a problem when you watch TV shows at night and want to avoid the huge difference in volume between whispers and war scenes. To optimize the overall loudness of the current program, the audio processor can provide a variety of dynamic range compression options, from wideband to multi-band.
Higher power level
Flat-panel TVs with large displays typically have a larger viewing distance than CRT TVs, requiring more audio power. In addition, flat-panel TVs lack the cooling space required for Class AB amplifiers. These two factors make digital audio amplifiers a better choice for flat-panel TVs.
There are two basic types of digital audio amplifiers. A conventional Class D amplifier receives an analog input signal and uses an analog circuit to generate a pulse width modulated switching output. Traditional Class D amplifiers are ideal for products with analog sources, but digital Class D amplifiers are better suited for products with digital audio sources such as digital TV.
Digital Class D amplifiers accept digital audio inputs and use digital circuitry to generate switching outputs, eliminating the need for a DAC at the audio processor output. Digital amplifiers have a high signal-to-noise ratio and dynamic range because the signal path is fully digitized. In addition, audio processing can be integrated into the amplifier if additional processing power is required.
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