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Digital Voice recorder


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#16 APGBryan

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Posted 10 November 2009 - 12:14 AM

[That is very informative and I appreciate the exhaustive reply. I understand what you are saying regarding not knowing the elements of the evp, etc. I had never really considered this, but I was under the impression that you had set those standards to eliminate the possibility of false positives. If one wanted to be confident that what they are capturing is a true representation of "actual sound" in order to avoid false positives, shouldn't 48 kps be sufficient?


Two factors would need to be considered if you use lower sample rates

First, of course is that you are aware of and have no intention beyond simply listening. With that in mind you could use 48KBPS.

Second and more important, you will need to record through a bandpass filter to block anything over 8 kHz. Reason is that should an insufficient sampling rate be used, based on the audio frequency, the possibility of aliasing increases dramatically. This can result in secondary sounds in the lower frequency regions as the sample rate hetrodynes with the higher audio frequency components. Thus these will need to be blocked to prevent them from entering the recorder'

As to the sound quality You may notice a slight difference between 48 KBPS and 96 KBPS, but not appreciably. When I conducted my tests here I used simulated EVPs containing audio components up to 15 kHz. The tests were conducted using a Fostex FR2 digital recorder. This recorder can sample up to 192 KBPS, but can also be dongraded to lower rates. One could see no appreciable difference between 192KBPS and 96 KBPS. on a scope nor could you hear any. When you drop down to 48KBPS you could observe a 3 db rolloff at 15 Khz over the 96 KBPS. Listening wise it was barely noticable. Not serious but that was were it first became apparent. Below that it was much greater and could be easily heard as well. Thus my resoning for setting these values. In a pinch 48 KBPS should work OK. Certainly better than the 8 to 14 K that most digital voice recorders use!


Thanks. I appreciate the time you took to respond.

#17 IowaJenny

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Posted 14 November 2009 - 11:17 PM

I have a zoom h4n but it never gets used, I have a free recorder app on my iphone that has picked up waaaayyyy more EVPs for some reason.

I am going to have to check that app out. I have no equiptment with exception to the iPhone. Thanks!

#18 afterlife

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Posted 27 January 2010 - 10:52 PM

I have a zoom h4n but it never gets used, I have a free recorder app on my iphone that has picked up waaaayyyy more EVPs for some reason.


Of course you get more EVPs on a cheap recordeer. Trouble is they aren't really EVPs, rather simply artifacts caused by low sample rates, conversion errors, aliasing, IM distortion, and often RF interference from the poor shielding such recorders have.

I personally won't even consider a recording for EVP analysis unless it meets the following criterea:

1. Sample arte of 96 KBPS or greater

2. 24 bit A to D conversion

3. Stereo recording for redundency.

4. WAV or other uncompressed format, not CELP based. No MP3s. These can only be used to post a reduced quality EVP so someone can hear it, they should never be used for actual research.

Rec ordings made under these conditions don't need cleaned up, EVPs will be class A or they simply won't be there. About the only thing that needs done is some amplification to increase the volume level. EVPs digitally processed have been altered in content by the cleaning process (artifacts added or certain frequencies removed) and are useless for serious study.

Cave rat - What exactly do you mean by 24 bit A to D conversion? Is 44.1 kHz / 128kbps sufficient by your standards?

#19 CaveRat2

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Posted 28 January 2010 - 05:09 PM

Cave rat - What exactly do you mean by 24 bit A to D conversion? Is 44.1 kHz / 128kbps sufficient by your standards?

24 Bit A to D means that the analog to digital conversion uses 24 bits to represent the audio as opposed to 16 bits. This results in less interpolation when converting to digital. An analogy that might explain how this affects the sound is similar to comparing digital cameras and the number of pixels. A higher number results in a sharper, clearer picture. Obviously a 10 megapixel camera is capable of better images than a 4 megapixel. Likewise a 24 bit converter can differentiate more finely than a 16 (or fewer) bit conversion when dealing with audio.

128 KBPS would be acceptable if you are dealing with a non-lossy format (WAV, etc.). The mention of 44.1 khz is the limit often imposed by MP3 or audio CD formats. These are lossy in that there is compression applied and as such do alter the audio in doing so. If you are refering to just 44.1 khz as a frequency response from an uncompressed source it would be sufficient, however the use of any lossy format is not recommended regardless of its sample rate because of the compression issues. MP3 is not advised except for casual listening where no analysis is intended. (Forum posts for example)

#20 afterlife

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Posted 28 January 2010 - 06:24 PM

Cave rat - What exactly do you mean by 24 bit A to D conversion? Is 44.1 kHz / 128kbps sufficient by your standards?

24 Bit A to D means that the analog to digital conversion uses 24 bits to represent the audio as opposed to 16 bits. This results in less interpolation when converting to digital. An analogy that might explain how this affects the sound is similar to comparing digital cameras and the number of pixels. A higher number results in a sharper, clearer picture. Obviously a 10 megapixel camera is capable of better images than a 4 megapixel. Likewise a 24 bit converter can differentiate more finely than a 16 (or fewer) bit conversion when dealing with audio.

128 KBPS would be acceptable if you are dealing with a non-lossy format (WAV, etc.). The mention of 44.1 khz is the limit often imposed by MP3 or audio CD formats. These are lossy in that there is compression applied and as such do alter the audio in doing so. If you are refering to just 44.1 khz as a frequency response from an uncompressed source it would be sufficient, however the use of any lossy format is not recommended regardless of its sample rate because of the compression issues. MP3 is not advised except for casual listening where no analysis is intended. (Forum posts for example)

Ok, I understand about how the higher bit rate is better, I just wasn't sure exactly what the A to D thing was. My recorder is in wav format & all I could find on the spects were the 44.1kHz / 128kbps. I'm just trying to make sure my recorder doesn't need to be up graded. Thanks for your help..!




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