Television – Format – Including additional information
Reexamination Certificate
1999-01-11
2001-07-10
Kostak, Victor R. (Department: 2611)
Television
Format
Including additional information
C348S484000, C381S002000
Reexamination Certificate
active
06259482
ABSTRACT:
FIELD OF INVENTION
This application relates to signal conditioning system, and more particularly to a digital compander system that is compatible with the FCC's approved BTSC encoding standards.
BACKGROUND OF INVENTION
On Mar. 29, 1984, the FCC Report and Order in Docket No. 21323 adopted and authorized a standard for multichannel television sound (MTS). Bulletin No. 60 of the Office of Science and Technology, “Multichannel Television Sound Transmission and Audio Processing Requirement for the BTSC System” (OST 60) contains the technical specifications for the above mentioned MTS adopted by the FCC.
The MTS system, known variously as MTS, BTSC stereo, or broadcast TV stereo, consists of two parts: an overall transmission system, and a noise reduction, or companding system. The “BTSC System Multichannel Television Sound Recommended Practices” (BTSC Standard) defines companding as: “a noise reduction process used in the stereophonic subchannel and in the second audio program subchannel consisting of compression (encoding) before transmission and complementary expansion (decoding) after reception. (This definition conforms to “companding” in OST 60 Section A).”
Without companding, the transmission system is capable of delivering high-quality 5 stereophonic audio. However, FM-transmission systems experience squared relationship between noise and frequency (Parabolic Noise Characteristic) resulting in higher noise at the higher frequencies. In addition, to avoid causing interference, the BTSC Standard limits the amount of modulation that can be applied to the stereophonic signal. Thus, even under ideal conditions, the addition of the subcarrier adds approximately 15 dB of noise to stereophonic (stereo) reception compared to monophonic (mono) reception. To make matters worse, under certain impaired transmission/reception conditions, such as a weak received signal, transmitter ICPM and multipath effects, buzz or hum can be introduced onto the transmitted audio. Thus, without companding, the service area for stereophonic TV (stereo-TV) reception is smaller than for monophonic TV (mono-TV) reception.
The situation is worse for the Second Audio Program (SAP) channel. The SAP subcarrier frequency is 78.7 KHz, much higher than for the stereo signal; therefore, the Parabolic Noise Characteristic results in even more noise in the signal. Furthermore, this subcarrier uses FM, which is additionally subject to picture-to-audio intermodulation (buzz beat), causing a particularly annoying distortion.
The BTSC noise-reduction system was designed to be a cost-effective aid to the MTS transmission system in delivering a clean, noise-free audio signal into the home. Specifically, the system was designed to: provide significant noise reduction even in poor reception areas while preserving input-signal dynamic range; prevent the stereo-subcarrier from interfering with overall transmitted power levels (AM-interleave effects); ensure reliable performance even in the face of manmade noise and transmission/reception-system impairments; and to provide these benefits at a commercially reasonable cost.
To achieve the above-stated goals, the BTSC Standard departed from previous design approaches where the dynamic range of the impaired channel was very low. In the case of the stereo-subcarrier in Grade B reception, the available dynamic range was about 43 dB, while in the SAP channel the dynamic range was about 26 dB. These compare unfavorably with the typical dynamic range of a compact cassette at about 60 dB. The significance of these figures, the operation of prior art compander systems, and the operation of the current invention will be better understood after a discussion of the psychoacoustic phenomenon of masking.
All audio noise-reduction systems work on the principle of masking; a listener will be oblivious to the noise on a transmission when the program signal, music or speech, is loud enough and its spectral content is broad enough, to mask the noise.
For example, if the program consists of low-frequency sounds, it must be transmitted at a high level relative to the background noise of the stereo-subcarrier channel to capture the listener's attention and for the listener to be unaware of the background noise. On the other hand, a broadband signal does not need to be much higher in amplitude than the background noise for the noise to fade below the listener's perception threshold. See, for example, I. M. Young, and C. H. Wenner,
Masking of White Noise by Pure Tone, Frequency
-
Modulated Tone, and Narrow
-
Band Noise
, J. Acoust. Soc. Am. 41, pp. 700-705, 1967.
The noise reduction system must compress and encode the audio signal such that it will consistently mask the channel noise during transmission, and then decode and expand the transmitted signal to recover the original audio signal. In passing through the encode/decode (companding) cycle, distortion or other degradation of the audio signal must be kept to a minimum. And in the decoding process, all the audible noise should be eliminated. Thus, not only must the level of the transmitted audio be high relative to the background noise, but the frequency of the signal and noise must be considered when selecting or designing an effective noise masking and companding scheme. Other characteristics of the signal and noise will affect the design of an ideal companding system. Thus, the amplitude of the signal, the rate at which the signal changes amplitude, and even whether the signal is decreasing or increasing are also important parameters in the design of an ideal companding system.
The stereo-subcarrier's background-noise spectrum is white, rising at 3 dB-per-octave. By comparison, the SAP subcarrier noise rises at 9 dB-per-octave. The noise can be masked if the transmitted signal's spectrum contains substantial high-frequency, especially in the case of the SAP channel. If so, the compander would only have to keep the signal amplitude levels high through the transmission medium. However, most TV program materials have their dominant energy at low frequencies. Alternatively, if the program consistently lacked high-frequency content, one could simply apply a constant rising preemphasis characteristic to the entire audio spectrum.
However, today's TV program content, especially music and movie effects, is neither consistently high frequency nor low frequency, for either approach to work. Inevitably, the signal will have instances of high-level, high-frequency energy in the audio signal, where a constant rising preemphasis would cause headroom (overload distortion) problems.
Existing solutions to the problem of preserving psychoacoustic masking and simultaneously preserving headroom at all frequencies use spectral companding, a preemphasis scheme which adapts its characteristic to suit the signal. The spectral compressor in the encoder measures the spectral balance of the input signal and varies the high-frequency-preemphasis accordingly, merely increasing the potential for masking, and reducing the possibility of high-frequency overload. The resulting encoded signal is, therefore, dynamically adjusted to consistently contain a substantial proportion of high frequencies before transmission, thereby masking the channel noise.
During reception, the spectral expander (in the decoder) restores the high frequencies to their proper amplitude. If the original input signal contains predominantly low frequencies, the decoder attenuates the high-frequency background noise, leaving the low-frequency signal and low-frequency background noise, the latter of which is masked by the signal itself. If the original input signal contains predominantly high frequencies, the decoder does not attenuate the high frequencies to restore correct frequency response, since the signal itself masks the noise.
The spectral compressor achieves two simultaneous requirements: the system is forgiving of high-background-noise environments because the spectral shaping of the input signal is adjusted according to the needs of the input signal to provide high m
Darr Roger
Easley Matthew F.
Boss Gerald R.
Kostak Victor R.
Troutman Sanders LLP
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