File Size is increasing, when saving new mf4 file

[gual11]

We work with existing mf4 files, read out channels we need, modify the name and then write them into a new mf4 file. We work with compression= 1, mostly that works. Sometimes the file increases a little in size, which is ok. But sometimes file size is 5 to 10 times as huge as the original one. With a 500 MB file that results in some GB, which is a lot.

Can you tell, why this happens sometimes? Is it because of some signal format? actually because we get rid of some signals, we would expect the file size to be even lower than the original one.

[gual11]

MDF Version 4.10
asammds Version 7.4.5

[danielhrisca]

Integer signals that are not a standard bit size will be saved as standard size ( 1 bit signal becomes 8 bit, 10 bit signal becomes 16 bit ).

Another increase is caused when VLSD signals are stored (variable length signal) because the biggest sample length will be used by the saved signal.

You should use compression=2 to get extra space saving

[gual11]

thaks for the quick answer.
Even with comprassion = 2 we faced the same behaviour.
Regarding these VLSD signals, how do I identify them and can I compress them or influence their save sample size?

[danielhrisca]

Can you share how you create the new MDF ?

[gual11]

mdf = asammdf.MDF()
for partnumber,subdict in data.items():
	sigs = []
	for channelname,channeldata in subdict.items():
		try:
			signal = asammdf.Signal(samples=channeldata[:,1],timestamps=channeldata[:,0]*10**(-9),name=channelname,comment=partnumber)
			sigs.append(signal)
		except Exception as e:
			print(channelname,e)
	mdf.append(sigs,acq_name=partnumber)
if status:
	mdf.header['abs_time'] = offset
	mdf.save(path2export,overwrite=True,compression=1)
mdf.close()