for M = [4]
    bits = Signalgenerator("form", signalform.prms,"M", M,"order", 18).process();
    mapper = PAMmapper(M,0);
    symbols = mapper.map(bits);

    db = Duobinary();
    pr1 = Partialresponse();

    symbols_pre = db.precode(symbols);
    symbols_db = db.encode(symbols_pre);

    %%% TEST 1
    symbols_db_ = awgn_channel(symbols_db,"snr_dB",15);
    figure(1003042);
    showLevelHistogram(symbols_db_,symbols_db,"ref_symbol_uncoded",symbols);
    showLevelHistogram(symbols_db_,symbols_db);

    symbols_rx = db.decode(symbols_db);
    bits_rx = PAMmapper(M,0).demap(symbols_rx);
    [~,~,ber,~] = calc_ber(bits.signal,bits_rx.signal,"skip_front",0,"skip_end",0,"returnErrorLocation",1);
    disp(['Class BER: ',sprintf('%.1E',ber),' - PAM-',num2str(M)]);

    symbols_pre_pr = pr1.precode(symbols,"M",M);
    symbols_db_pr = pr1.encode(symbols_pre_pr,"M",M);
    
    %%% TEST 2
    symbols_db_pr_ = awgn_channel(symbols_db_pr,"snr_dB",15);
    figure(1003043);
    % showLevelHistogram(symbols_db_pr_,symbols_db_pr,"ref_symbol_uncoded",symbols);
    
    symbols_rx_pr = pr1.decode(symbols_db_pr,"M",M);
    bits_rx_pr = mapper.demap(symbols_rx_pr);
    [~,~,ber_pr,~] = calc_ber(bits.signal,bits_rx_pr.signal,"skip_front",0,"skip_end",0,"returnErrorLocation",1);
    disp(['Partialresponse BER: ',sprintf('%.1E',ber_pr),' - PAM-',num2str(M)]);

    pre_match = isequaln(symbols_pre.signal, symbols_pre_pr.signal);
    enc_match = isequaln(symbols_db.signal, symbols_db_pr.signal);
    dec_match = isequaln(symbols_rx.signal, symbols_rx_pr.signal);

    pre_maxdiff = max(abs(symbols_pre.signal - symbols_pre_pr.signal));
    enc_maxdiff = max(abs(symbols_db.signal - symbols_db_pr.signal));
    dec_maxdiff = max(abs(symbols_rx.signal - symbols_rx_pr.signal));

    disp(['Precode match: ',num2str(pre_match), ...
        ' | max abs diff: ',sprintf('%.3g',pre_maxdiff)]);
    disp(['Encode  match: ',num2str(enc_match), ...
        ' | max abs diff: ',sprintf('%.3g',enc_maxdiff)]);
    disp(['Decode  match: ',num2str(dec_match), ...
        ' | max abs diff: ',sprintf('%.3g',dec_maxdiff)]);
    disp('---');



    f = figure("Name",sprintf("PAM-%d Partial-Response Histograms",M));
    t = tiledlayout(f,1,4,"TileSpacing","compact","Padding","compact");
    title(t,sprintf("PAM-%d Symbol Histograms",M));

    nexttile;
    plotDiscreteHistogram(symbols.signal,sprintf("PAM-%d",M));

    nexttile;
    plotDiscreteHistogram(symbols_db_pr.signal,"Duobinary 1st Order");

    nexttile;
    plotDiscreteHistogram(getEncodedSymbols(symbols,M,2),"Duobinary 2nd Order");

    nexttile;
    plotDiscreteHistogram(getEncodedSymbols(symbols,M,3),"Duobinary 3rd Order");

end

function y = getEncodedSymbols(symbols, M, order)
    pr = Partialresponse("order",order);
    y = pr.encode(pr.precode(symbols,"M",M),"M",M).signal;
end

function plotDiscreteHistogram(x, plot_title)
    levels = unique(x(:)).';
    counts = zeros(size(levels));

    for idx = 1:numel(levels)
        counts(idx) = sum(abs(x - levels(idx)) < 1e-12);
    end

    bar(levels, counts, 0.9, "FaceColor", [0.2 0.45 0.75], "EdgeColor", "none");
    grid on;
    box on;
    xlabel("Level");
    ylabel("Count");
    title(plot_title);
    xticks(levels);
end