Why do I receive an "Unrecognized field name 'SampleRate'" error when using "writeall" with MP4 files in R2021b?

I am attempting to use the "writeall" function with the "OutputFormat" as MP4. I am receiving the following error:

Error using matlab.io.datastore.FileDatastore/writeall (line 482)

Unrecognized field name "SampleRate".

My code is as follows:

fds = fileDatastore('myvideo.mp4','ReadFcn',@readfcn,'FileExtensions','.mp4');
location = 's3://mybucket/output/';
writeall(fds,location,'OutputFormat','mp4')

function data = readfcn(file)
    data = importdata(file);
end

What is causing this error?

Answer 1

[MathWorks Support Team]

The "writeall" function requires the audio sampling rate in order to write with the MP4 output format. You can specify the "SampleRate" field using a custom write function:

 

fds = fileDatastore('myvideo.mp4','ReadFcn',@readfcn,'FileExtensions','.mp4');
location = 's3://mybucket/output/'; 
writeall(fds,location,'WriteFcn',@writefcn)

function data = readfcn(file)
    data = audioread(file);
end

function writefcn(data,writeInfo,outputType)
    sampleRate = 44100;
    audiowrite(writeInfo.SuggestedOutputName,data,sampleRate);
end