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fix(matlab): correct BS67BW Fourier frequencies #17

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fix(matlab): correct BS67BW Fourier frequencies #17

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3 changes: 2 additions & 1 deletion echoframe/matlab/core/imaging/echoframe_setup_fourier.m
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Original file line number Diff line number Diff line change
Expand Up @@ -134,7 +134,8 @@
case 'BS67BW'
Nz = single(ReceiveSpec.nSamples * 2);
freq_scale = 2;
lower = Nz/2;
% The negative-frequency half starts after the positive half in FFT order.
lower = Nz/2 + 1;
freq_window = lower:(lower + Nz/4 - 1);
freq_mapping = 1:Nz/4;
freq_axis_shift = false;
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3 changes: 2 additions & 1 deletion echoframe/matlab/core/verasonics/get_system_parameters.m
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Expand Up @@ -14,7 +14,7 @@
%
% nChannels 128 or 256, from Resource.Parameters.numRcvChannels
% and Trans.numelements
% Fs Trans.frequency * 4 (Hz)
% Fs pre-quadrature sample rate: 4*Fc, or 4*Fc/3 for BS67BW (Hz)
% samplesPerWavelength from samplingMode: 1 (BS50BW), 4/3 (BS67BW),
% 2 (BS100BW), 4 (NS200BW)
%
Expand Down Expand Up @@ -52,6 +52,7 @@
case 'BS50BW'
ReceiveSpec.samplesPerWavelength = 1;
case 'BS67BW'
ReceiveSpec.Fs = Trans.frequency * (4/3) * 1e6;
ReceiveSpec.samplesPerWavelength = 4/3;
case 'BS100BW'
ReceiveSpec.samplesPerWavelength = 2;
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54 changes: 54 additions & 0 deletions echoframe/matlab/tests/verify_core_headless.m
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Expand Up @@ -156,6 +156,8 @@
assert(threw, 'validate_structs accepted a ReceiveSpec missing samplingMode');
fprintf(' missing required field rejected OK\n');

local_verify_fourier_sampling();

%% ------------------------------------------------------- 5. cropped frames
fprintf('\n=== stored_frame_size (cropped storage geometry) ===\n');

Expand Down Expand Up @@ -490,3 +492,55 @@
PDISpec.cropPDI = logical(false);
PDISpec.svdMethod = 'Covariance';
end

function local_verify_fourier_sampling()
% A known transmit delay must be cancelled at the dominant receive-tone bin.
modes = {'BS50BW', 'BS67BW', 'BS100BW', 'NS200BW'};
rateFactors = [4, 4/3, 4, 4];
P = struct('pitch', 300e-6, 'nElements', 8);
T = struct('c0', 1540, 'steer', 0, 'apodization', ones(8,1));
Resource = struct('Parameters', struct('numRcvChannels', 128));
for fc = [1e6, 31.25e6]
Trans = struct('frequency', fc/1e6, 'numelements', 8);
for n = [128, 512]
rec = struct('c0', 1540, 'nx', 128, 'nz', n/2);
for m = 1:numel(modes)
r = struct('samplingMode', modes{m}, 'nBuffers', 1, ...
'nSamples', n, 'nSamplesIQ', n/2, 'nTransmissions', 1);
[~, ~, ~, r] = get_system_parameters(Resource, Trans, T, r);
assert(abs(r.Fs/fc - rateFactors(m)) < 1e-6, ...
'Wrong pre-quadrature sample rate for %s', modes{m});
switch modes{m}
case 'BS50BW'
starts = 2*(0:n/2-1)/fc;
times = reshape([starts; starts + 1/(4*fc)], [], 1);
case 'BS100BW'
starts = (0:n/2-1)/fc;
times = reshape([starts; starts + 1/(4*fc)], [], 1);
case 'BS67BW'
times = (0:n-1)'*3/(4*fc);
case 'NS200BW'
times = (0:n-1)'/(4*fc);
end
for shift = [0, 1/(8*fc)]
T.transmitDelays = shift * ones(8,1);
s = echoframe_setup_fourier(P, T, r, rec);
assert(numel(s.frequencyAxis) == n/2);
assert(all(s.delayIndices(:) >= 0 & s.delayIndices(:) < n/2));
for tone = fc*[0.875, 1, 1.125]
a = int16(round(16000*cos(2*pi*tone*times)));
b = int16(round(16000*cos(2*pi*tone*(times-shift))));
a = fft(complex(single(a(1:2:end)), -single(a(2:2:end))));
b = fft(complex(single(b(1:2:end)), -single(b(2:2:end))));
[~, k] = max(abs(a));
rotation = exp(1i*double(s.frequencyAxis(k))*double(s.planewaveDelays(1,2)));
assert(abs(double(b(k))/double(a(k))*rotation - 1) < 2e-4, ...
'Fourier delay correction failed: %s, Fc=%g, n=%d, tone=%g', ...
modes{m}, fc, n, tone);
end
end
end
end
end
fprintf(' all four sampling modes cancel known tone delays OK\n');
end
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