4.写出一个函数由拨号音自动解码出电话号码matlab实现

以下是一个基本的 MATLAB 函数，它可以将拨号音自动解码为电话号码：

function phone_number = decode_dtmf(dtmf_signal, fs)
% DECODE_DTMF Decodes DTMF signals to phone number
%   PHONE_NUMBER = DECODE_DTMF(DTMF_SIGNAL, FS) decodes DTMF signals to 
%   phone number using the sampling frequency FS.
% 
%   Example usage:
%       dtmf_signal = audioread('dtmf_signal.wav');
%       fs = 8000;
%       phone_number = decode_dtmf(dtmf_signal, fs);

% Define DTMF frequencies
frequencies = [697, 770, 852, 941, 1209, 1336, 1477];

% Define DTMF symbols
symbols = ['1', '2', '3', '4', '5', '6', '7', '8', '9', '*', '0', '#'];

% Define filter coefficients for each DTMF frequency
num_coeffs = 31; % number of filter coefficients
dtmf_filters = zeros(num_coeffs, length(frequencies));
for ii = 1:length(frequencies)
    dtmf_filters(:, ii) = fir1(num_coeffs-1, frequencies(ii)/(fs/2));
end

% Filter DTMF signal with each filter
filtered_signal = zeros(length(dtmf_signal), length(frequencies));
for ii = 1:length(frequencies)
    filtered_signal(:, ii) = filter(dtmf_filters(:, ii), 1, dtmf_signal);
end

% Find peaks in each filtered signal
peak_indices = zeros(length(frequencies), 2);
for ii = 1:length(frequencies)
    [peaks, locs] = findpeaks(abs(filtered_signal(:, ii)), 'MinPeakDistance', round(fs/frequencies(ii)/2));
    [max_peak, max_index] = max(peaks);
    peak_indices(ii, :) = [locs(max_index), ii];
end

% Sort peak indices by time
sorted_indices = sortrows(peak_indices);

% Decode phone number
phone_number = '';
last_index = 0;
for ii = 1:length(sorted_indices)
    if sorted_indices(ii, 1) - last_index > round(fs/20)
        % Time between peaks is greater than 50 ms
        phone_number = [phone_number, symbols(sorted_indices(ii-1, 2))];
    end
    last_index = sorted_indices(ii, 1);
end
phone_number = [phone_number, symbols(sorted_indices(end, 2))];
end

这个函数假设输入的是一个采样频率为 `fs` 的音频信号 `dtmf_signal`，其中包含了 DTMF 信号。函数首先定义了 DTMF 频率和符号的数组，然后根据每个 DTMF 频率计算出一个滤波器系数矩阵。之后，函数将输入信号与每个滤波器进行卷积，以得到每个 DTMF 频率的滤波输出。函数然后找到每个滤波输出的峰值，按时间顺序对它们进行排序，并将它们映射到符号。最后，函数将符号连接起来以形成电话号码。

请注意，这个函数是一个基本的实现，可能需要根据实际需求进行修改和改进。