How to improve bit reading process in Matlab?

2019 9 6
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更新时间：2020 7 18
8 次查看（30 天）

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I need to read a binary file and the Matlab coding is:
a = uint8(fread(fid,'ubit1')');
This is inefficient so I added a line below:
a = uint16(swapbytes(a));
But it doesn't help. Any other method to improve the bit reading process in Matlab?

14 个评论
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Walter Roberson
Walter Roberson 2019-9-6
Is there any reason that you are not reading uint8? And possibly extracting bits afterwards? Either with bitget() or with a lookup table ?
Star Rats
Star Rats 2019-9-6
When I tried:
a = (fread(fid,'uint8')');
It shows error: Index exceeds the number of array elements (3529175).
As my project is a decoder where each block contains different number of bit.
And I need to read the input file bit by bit in order to execute the function in each block.
Walter Roberson
Walter Roberson 2019-9-6
bittable = (dec2bin(0:255, 8) - '0').';
a8 = fread(fid, 'uint8');
a = reshape( bittable(a8, :), [], 1);
Star Rats
Star Rats 2019-9-6
Hi @Walter, I tried but I get an error: Index in position 1 is invalid. Array indices must be positive integers or logical values.
Walter Roberson
Walter Roberson 2019-9-6
编辑：Walter Roberson 2019-9-6
a = reshape( bittable(double(a8)+1, :), [], 1);
Star Rats
Star Rats 2019-9-6
I tried and it causes error: Index in position 1 exceeds array bounds (must not exceed 8).
So I tried:
a = reshape( bittable(double(a8), :), [], 1);
and it causes the previous error: Index in position 1 is invalid. Array indices must be positive integers or logical values.
Walter Roberson
Walter Roberson 2019-9-6
a = reshape( bittable(:,double(a8)+1), [], 1);
Star Rats
Star Rats 2019-9-6
Thanks @Walter. It runs without error. However, I wonder why it could not go into the loop of:
syncword = uint8([1 1 1 1 1 1 1 1 1 1 1 1 1 0 1]);
if(y(b:b+14) == syncword)
{
...
};
It could not scan if there exist the binary pattern of syncword.
Fyi, the coding I referring is here.
Walter Roberson
Walter Roberson 2019-9-6
Your sync_word is wrong. It should be
syn = ones(1,12);
isMP2 = 1; %0 for MP4
layer = zeros(1,2);
sync_word = [syn, isMP2, layer];
Same total number of bits, but you had 0 1 for layer instead of 0 0.
https://wiki.multimedia.cx/index.php?title=ADTS
Star Rats
Star Rats 2019-9-6
Hi @Walter, I think it is regardless of the syncword. It just couldn't go into the loop.
When I tested using the original coding, the if loop begins on the 1273th bit. But from the coding provided, it continues to scan the bit until the end without execute the if and for loop in between.
a = reshape( bittable(:,double(a8)+1), [], 1);
From the line above, is 'a' reading bit by bit or in an array form?
Walter Roberson
Walter Roberson 2019-9-6
It is reading byte by byte and translating to bits.
There is a theoretical possibility that the order of bits within a byte is reversed but it seems unlikely.
Star Rats
Star Rats 2019-9-12
Thanks @Walter, the code can be run but it does not improve the bit reading process. Is it possible to use the function GPUArray?
Walter Roberson
Walter Roberson 2019-9-12
编辑：Walter Roberson 2019-9-12
Yes, you can use gpu array. However it will slow down your code.
Star Rats
Star Rats 2020-7-18
Thanks @Walter for your help. You are definitely a Matlab magician!

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