authMultimediaAssignment2018/Level_1/SSC.m

function frameType = SSC(~, nextFrameT, prevFrameType)
%Implementation of the Sequence Segmentation Control step
%   Usage frameType = SSC(frameT, nextFrameT, prevFrameType), where:
%       Inputs
%       - frameT is a frame in the time domain, containing both channels of
%           the audio stored in an array of dimensions 2048X2
%       - nextFrameT is the next frame in the time domain, containing both
%       channels of the audio stored in an array of dimensions 2048X2
%       - prevFrameType is the type of the previous frame in string
%       representation, can be one of "OLS" (ONLY_LONG_SEQUENCE), "LSS"
%       (LONG_START_SEQUENCE), "ESH" (EIGHT_SHORT_SEQUENCE), "LPS"
%       (LONG_STOP_SEQUENCE)
%
%       Output
%       - frameType is the type of the current frame in string
%       representation, can be one of "OLS" (ONLY_LONG_SEQUENCE), "LSS"
%       (LONG_START_SEQUENCE), "ESH" (EIGHT_SHORT_SEQUENCE), "LPS"
%       (LONG_STOP_SEQUENCE)

    % Examines the cases where the determination of the type of the next
    % frame isn't needed for
    if strcmp(prevFrameType, 'LSS')
        frameType = 'ESH';
        return;
    elseif strcmp(prevFrameType, 'LPS')
        frameType = 'OLS';
        return;
    end

    % Determines the type of the next frame
    % Filters frame
    nextFrameT = filter([0.7548, -0.7548], [1, -0.5095], nextFrameT, [], 1);

    channelFrameType = {'nan', 'nan'};

    for channel = 1:2
        % Calculates sub-frame energy estimation
        [subFrames, ~] = buffer(nextFrameT(449:end - 448, channel), 256, 128, 'nodelay');
        energyEstimations = sum(subFrames .^ 2, 1);

        % Calculates the ratio of the sub-frame energy to the average energy of
        % the previous sub-frames
        energyRatios = movmean(energyEstimations, [8 0]);
        energyRatios = energyEstimations ./ [energyEstimations(1) energyRatios(1:end-1)];
        
        if ~isempty(find(energyEstimations > 10^(-3), 1)) && ...
                ~isempty(find(energyRatios(energyEstimations > 10^(-3)) > 10, 1))
            if strcmp(prevFrameType, 'ESH')
                % This frame of this channel is an EIGHT_SHORT_SEQUENCE type
                % frame. This means the frames of both channels will be encoded
                % as EIGHT_SHORT_SEQUENCE type frames.
                frameType = 'ESH';
                return;
            elseif strcmp(prevFrameType, 'OLS')
                channelFrameType{channel} = 'LSS';
            end
        else
            if strcmp(prevFrameType, 'ESH')
                channelFrameType{channel} = 'LPS';
            elseif strcmp(prevFrameType, 'OLS')
                channelFrameType{channel} = 'OLS';
            end
        end
    end

    if strcmp(channelFrameType{1}, 'nan') || strcmp(channelFrameType{2}, 'nan')
        error('SSC, l[73]: Internal error occured!')
    end
    if strcmp(channelFrameType{1}, 'OLS')
        frameType = channelFrameType{2};
    elseif strcmp(channelFrameType{2}, 'OLS')
        frameType = channelFrameType{1};
    elseif strcmp(channelFrameType{1}, channelFrameType{2})
        frameType = channelFrameType{1};
    else
        frameType = 'ESH';
    end
end