# US 5,703,954 A · Method and apparatus for improving the quality of AM compatible digital broadcast system signals in the presence of distortion > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,703,954 A - Document ID: US5703954A - Status: Granted - Kind: A - Application number: 08/604,199 - Filed: 1996-02-20 - Granted: 1997-12-30 - Earliest claimed: 1996-02-20 - Assignees: USA Digital Radio Partners, L.P. - Inventors: Mark J. Dapper; Barry W. Carlin; Michael J. Geile - Web page: https://patentgrep.com/patent/US5703954A ## CPC - H04H 40/18: Arrangements characterised by circuits or components specially adapted for receiving - H04H 20/22: Arrangements for broadcast of identical information via plural broadcast systems - H04H 20/26: Arrangements for switching distribution systems - H04H 20/33: by plural channels - H04H 2201/186: AM digital or hybrid - H04H: BROADCAST COMMUNICATION (multiplex communication H04J; pictorial communication aspects of broadcast systems H04N) ## Abstract A system and method are provided for reducing perceived distortion in an output audio signal derived from an amplitude modulated compatible digital broadcast signal having a monophonic portion and a stereo portion. The stereo portion of this signal is demodulated to produce a demodulated stereo signal and the monophonic portion of the signal is demodulated to produce a demodulated monophonic signal. The noise in the demodulated stereo signal is measured and an estimate of the signal-to-noise ratio is derived. This estimated signal-to-noise ratio is compared with a threshold level which is set at the minimum acceptable signal quality for the stereo signal. Based upon the estimated signal-to-noise ratio, a switch directs either the monophonic derived audio output or the stereo derived audio output as the system output signal. ## Claims ### Claim 1 A system for reducing perceived distortion in an output audio signal derived from an amplitude modulated compatible digital broadcast signal having a monophonic portion and a stereo portion, said system comprising: (a) means for demodulation said monophonic portion of said signal to produce a demodulated monophonic signal; (b) means for demodulation said stereo portion of said signal to produce a demodulated stereo signal having a signal-to-noise ratio; (c) means for estimation the signal-noise ratio in the demodulated stereo signal, said means for estimating the signal-noise ratio in the demodulated stereo signal comprising means for estimating the noise in the demodulated stereo signal, means for estimating the power of the demodulated stereo signal, and means for dividing the estimated noise in the demodulated stereo signal into the estimated power of the demodulated stereo signal, wherein said means for estimating the noise in the demodulated stereo signal comprises means for subtracting the demodulated stereo signal from said stereo portion of said amplitude modulated compatible digital broadcasting signal to obtain an error estimate, means for squaring said error estimate, and means for averaging said squared error estimate; (d) means for comparing the estimated signal-to-noise ratio in the demodulated stereo signal to a threshold level set at the minimum acceptable signal quality; and (e) means responsive to said means for comparing for synthesizing discrete left and right stereo outputs. ### Claim 2 (depends on 1) The system of claim 1 further comprising: (f) means for decoding said demodulated monophonic signal into a first signal; and (g) means for decoding said demodulated stereo signal into a second signal, wherein said first and second signals are inputs to said means for synthesizing discrete left and right stereo outputs. ### Claim 3 (depends on 1) The system of claim 1 wherein said first and second signals are analog signals. ### Claim 4 (depends on 1) The system of claim 1 wherein said means for synthesizing discrete left and right stereo outputs includes means for combining relative portions of said first and second analog signals in response to said means for comparing. ### Claim 5 (depends on 1) The system of claim 1 wherein said means for estimating the power of the demodulated stereo signal comprises means for squaring the demodulated stereo signal and means for averaging said squared signal. ### Claim 6 (depends on 1) The system of claim 1 wherein said means for estimating the signal-to-noise ratio in the demodulated stereo signal detects a monotonic variation in a quantity of said demodulated stereo signal as a function of the mean and variance of said quantity. ### Claim 7 (depends on 1) The system of claim 1 wherein said means for synthesizing discrete left and right stereo outputs operates by employing hysteresis. ### Claim 8 A method for reducing perceived distortion in an output audio signal derived from an amplitude modulated compatible digital broadcast signal having a monophonic portion and a stereo portion comprising the steps of: (a) demodulating said monophonic portion of said signal to produce a demodulated monophonic signal; (b) demodulating said stereo portion of said signal to produce a demodulated stereo signal having a signal-to-noise ratio; (c) estimating the signal-to noise ratio in the demodulated stereo signal by estimating the power of the demodulated stereo signal, and dividing the estimated noise in the demodulated stereo signal into the estimated power of the demodulated stereo signal, wherein the noise in the demodulated stereo signal is estimated by subtracting the demodulated stereo signal from said stereo portion of said amplitude modulated compatible digital broadcasting signal to obtain an error estimate, squaring said error estimate,and averaging said squared error estimate; (d) comparing the estimated signal-to-noise ratio in the demodulated stereo signal to a threshold level set at the minimum acceptable signal quality; and (e) synthesizing discrete left and right stereo outputs in response to said comparison of the estimated signal-to-noise ratio in the demodulated stereo signal to a threshold level set at the minimum acceptable signal quality. ### Claim 9 (depends on 8) The method of claim 8 further comprising the steps of: (f) decoding said demodulated monophonic signal into a first signal; and (g) decoding said demodulated stereo signal into a second signal, wherein said first and second signals are input to synthesize said discrete left and right stereo outputs. ### Claim 10 (depends on 8) The method of claim 8 wherein said first and second signals are analog signals. ### Claim 11 (depends on 8) The method of claim 8 wherein relative portions of said first and second analog signals are combined to synthesize said discrete left and right stereo outputs includes means in response to the comparison of the estimated signal-to-noise ratio in the demodulated stereo signal to a threshold level set at the minimum acceptable signal quality. ### Claim 12 (depends on 8) The method of claim 8 wherein the step of estimating the power of the demodulated stereo signal comprises the steps of squaring the demodulated stereo signal and averaging said squared signal. ### Claim 13 (depends on 8) The method of claim 8 wherein the step of the signal-to-noise ratio in the demodulated stereo signal comprises the step of detecting a monotonic variation in a quantity of said demodulated stereo signal as a function of the mean and variance of said quantity. ### Claim 14 (depends on 8) The method of claim 8 wherein the step of synthesizing discrete left and right stereo outputs operates by employing hysteresis.