Choosing a Field Recorder for Anurans - A Primer

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chrish
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Choosing a Field Recorder for Anurans - A Primer

Post by chrish »

** First off, let me start with the caveat that I am not a sound engineer or audio junkie. I am a herper who has taught himself a little bit about recording amphibian calls and wanted to share some of what I have learned through trial....and lots of error. CH

Recording calling anurans presents some challenges that normal wildlife recorders don’t face. It is usually done in wet areas or in water, in the dark, and often in the rain. You might be miles from your home or vehicle and have to carry everything with you.

So how do you choose a recorder for this task?

Of course, if budget is no concern and sound quality is the foremost consideration, you will probably end up with a high end field recorder such as the Sound Devices 722 and expensive microphones. But for most herpers with just a casual interest in documenting frog calls, that is probably overkill and it won’t fit in your pocket.

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Frequency Range

So the obvious first thought might be to turn to a Voice Recorder or use an app on your cell phone or tablet. But voice recorders, such as those in most phones, are optimized for capturing the human voice in its normal frequency ranges which is usually between 80 and 2000 Hz.. Some inexpensive voice recorders therefore only record frequencies up to about 4000Hz (4 kHz). The frequency cutoff depends on the particular mode you use in your recorder. For example, one common entry level voice recorder made by Olympus only records frequencies up to 3 kHz in certain space saving modes.

While this is plenty to record the sounds of the human voice, it may not accurately capture some anurans that call outside of this range. Several species of Eleutherodactylus, for example, have very high pitched calls that can be in excess of 8 kHz. The North American Little Grass Frog (Pseudacris ocularis) calls are in the 7.5 kHz range. Even frogs whose carrier frequency (main note frequency) is below the 8 kHz threshold, the actual “sound” of their call is dependent on higher sideband or harmonic frequencies which can be well above the carrier frequency.

A good example of a North American anuran call in which the sidebands/harmonics influence the overall sound of the call is the Ornate Chorus Frog (Pseudaris ornata). The following is a recording of a Pseudacris ornata taken from somewhere out on the world wide web (sorry, I don’t remember where!). I took a short section of the recording and copied it. The first time it plays, it is playing at “CD quality” sample rate of 44.1 kHz (with a maximum frequency of 22 kHz). It then repeats having the sample rate reduced to 8 kHz (max frequency of 4 hKhz). Here is a sonogram of what is in this recording. The highlighted calls in the first recording are all that is left in the second. All the higher parts of the call are lost.

Image

You can hear the difference the loss of these over tones makes:



When you listen to the recording, the second time through, it sounds distinctly different. This is due to the loss of the harmonics/sidebands above 4 kHz. So if this frog had been recorded with a low end voice recorder at a high compression setting, the call would sound like the second part of the recording. While that is certainly enough to identify the species in question, it clearly loses some of the texture or tone of this particular species’ call.

So a dependable frog recorder needs to be able to capture the range of frequencies used by most anurans so your recordings be representative of what the frog sounded like in the field. But you have to pay attention when evaluating the potential frequency range a recorder can capture. Many recorders describe their potential frequency range not by the raw frequencies captured but by their sampling rate. A recorder may list itself as having a sampling rate of 48 kHz. For practical applications, a recorder can capture sounds that are ½ of the sampling rate. So a recorder with a sampling rate of 48 kHz can capture frequencies up to 24 kHz. CD-roms have a sample rate of 44.1 kHz and so some recorders are advertised as being capable of “CD quality sound” or some similar description. At that sample rate, they will record frequencies up to 22.5 kHz which is plenty for almost all anuran calls. There is a good, but somewhat technical, discussion of sampling rate and related topics on this PDF file from Cornell Bioacoustics Lab - http://www.birds.cornell.edu/brp/pdf-do ... lSound.pdf

There are anurans known to call above these frequencies, but those ultrasonic frequencies are above the range of human hearing anyway so probably aren’t of interest to most casual anuran recordists. If you were interested in ultrasonic calls, there are recorders that go well beyond this. Some recorders can capture ultrasound with sampling rates of 96kHz, 192kHz or even 384kHz but they generally require specialized ultrasonic microphones to do so. While these recorded “sounds” can’t be heard by the human ear, they can have their frequencies brought down into human hearing range after recording. This is how bat biologists record and analyze bat calls.

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Signal to Noise Ratios

A microphone works by picking up the vibrations in the air generated by the sound wave and generates a weak electrical current corresponding to that sound wave. However, that current is so weak that in order for the signal to produce an audible recording, it has to be amplified before it is recorded. This is called “preamplification” and most recorders have built in “preamps” for this purpose.

This introduces a new problem. I the circuitry isn’t built very carefully, the process of preamplification can create sound (noise) in the final recording. Cheaper preamp circuits produce more electronic noise than higher quality preamps. When dealing with loud sounds like guitar music, drums, people talking close to the microphone, etc., it isn’t an issue as the signal is so loud that the additional preamp noise is inconsequential. But when recording natural sounds like frog calls, this amount of added noise can really interfere in the ability to discriminate quieter frog calls. Good recorders have quieter preamplifiers.

Here is a comparison of the two species of anurans (Hyla chrysoscelis and Incilius nebulifer) recorded simultaneously from the same spot with two different recorders. One is recorded with an Motorola Android cell phone and the other with an Olympus LS-11 Digital PCM recorder. The Olympus recorder has good preamps and produces less noise than the Android phone. You can actually see this in the sonogram for this recording.

Image

On the top recording (the phone) the background is much darker. This dark background represents noise in the recording. In the bottom sonogram, the background is much ligher while the frog calls are still as dark. Therefore you can see there is more signal (the darkness of the frog calls) compared to the noise (darkness of the background).

Here is a shorter section of these recordings played one after the other. In the first part, you hear the phone recording followed by a second of silence then the recording made by the Olympus LS-11. Listen to the background hiss in both recordings and compare how well the calls stand out. It is easiest to hear in the short buzzy trills of the Cope’s Gray Treefrogs (Hyla chrysoscelis). The longer trill is a Gulf Coast Toad (Incilius nebulifer).



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Recording Format

Another consideration is the format in which the recording will be captured. In voice recorders, a selling point is often the maximum number of hours of recording which can be stored in the recorder. In order to maximize that number the recordings may be compressed into lossy formats and the frequency compressed into the range expected for the human voice. This means higher frequency calls may be lost or significantly degraded in the final recording. When choosing a recorder, it is preferable to have a recorder which will save the file in an uncompressed format (aiff, wav) rather than a compressed format (mp3).

There is a nice comparison showing the limitations of voice recorders here - http://www.wildlife-sound.org/equipment ... chrec.html

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Other Practicalities

Beyond the technical capabilities, we need a recorder that is small, field hardy, somewhat weather-resistant. Furthermore, it is preferable to have a recorder that is easy to adjust and monitor in the field. Some recorders require you to go down through menus to make simple changes like the input level (gain) of the microphone. That can hard to do when you are out in the field, knee deep in water in the dark. Also, since you might be holding a microphone in the other hand, it can be helpful to be able to make changes with one hand.

Storage media and connectivity

How the recorder stores its recordings is another important consideration. While older handheld recorders relied on cassetes or microcassetes, the noise generated by those recorders made them obsolete with the advent of digital storage. Some recorders store their recordings on internal flash memory while others rely on removeable media such as SD cards. Some older models use compact flash cards and others rely on internal hard drives. These methods are OK, but not as field hardy as more modern methods.

Getting the recordings off the recorder is easy with most modern recorders. They either have removeable cards which can be put into a computer and/or they have mini USB ports plugs on the side to allow direct connection. Either way, getting recordings into your computer is a breeze.

Microphones

Another variable to consider when purchasing a recorder is what types of external microphones it will accept. There are two primary microphone plug types used in recorders. Inexpensive microphones and recorders use a 3.5mm TRS plug similar to a headphone plug on an MP3 player. These plugs as small and easy to use. There are inexpensive adapters and extension cords available for these sized cords at almost any electronics stores and even many Wal-mart type stores. Microphones with these types of plugs usually require a battery to power the microphone which is in the microphone itself.

Professional microphones generally have a 3 pin XLR type plug instead. These plugs are larger and many of the XLR plug microphones depend on power to be supplied by the recorder itself. This recorder-based power supply is often called phantom power. Some recorders are capable of providing phantom power and some aren’t. Phantom power voltages vary from microphone to microphone but most recorders that supply phantom power can supply it at various voltages.

So why would you go the trouble of using an phantom powered XLR type microphone when a 3.5mm microphone would be easier?

One difference is the sturdiness of the connection. TRS pins can become unplugged easily if pulled. I have on more than one occasion been recording with my 3.5mm plug microphone only to find out it wasn’t plugged in to the recorder and what I was actually recording with was the internal microphones of the recorder. TRS 3.5mm pins are also fairly thin and I have bent a couple when bumping (or dropping :-() my recorder when out in the field. XLR type connections are sturdier and often lock into the socket. In order to be pulled out, you have to depress a pin as you pull so accidentally pulling one out is harder.

The other difference is the quality of the sound. When a microphone has its own internal power supply and unbalanced inputs like many TRS 3.5 mm cables, that increases the chance that electrical interference will be produced in the line and show up on the recording as noise. With good microphones and cables this can be reduced, but it is never as quiet as a balanced phantom-powered, XLR type connection. The problem with noise in a recording is that you don’t notice it until you hear a recording that has less.

Part of the learning process is learning to hear the difference between a good (quiet) recording of an amphibian and a bad (noisy) one. Try going to online resources like SoundCloud.com and listening to some of the frog recordings on there. You will hear a profound difference in the quality based on the different recorders, microphones and techniques used.
You can also hear this difference by listening to the differences in the recordings used as vouchers in the HERP database. Unfortunately, at this time there is no method to search for just recorded vouchers, but you can visually look for the speaker icon next to a record to see if it contains a recording.

But most importantly, get outside, record some amphibians and have fun! And don't forget to upload your mp3 vouchers to the database!

Chris

fvachss
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Re: Choosing a Field Recorder for Anurans - A Primer

Post by fvachss »

Thanks for a very interesting post. Regarding the bandwidth limitation of typical pocket recording devices like cell phones, I wonder if heterodyning might permit the use of these devices to record higher frequency calls. Introduce a 20 KHz local oscillator and a even a small device with a 4 KHz bandwidth may record information in the 20 KHz frequency range (albeit with some post-processing required). Realistically though, I suspect that while this is technically feasible it's enough of a PITA that folks would do better just coughing up the money for a proper higher bandwidth recorder - much like the case in macro photography in which extension tubes are certainly usable but most folks are ultimately better served by the purchase of a dedicated macro lens (hey look, we're talking about photography again...)

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chrish
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Re: Choosing a Field Recorder for Anurans - A Primer

Post by chrish »

fvachss wrote:Thanks for a very interesting post. Regarding the bandwidth limitation of typical pocket recording devices like cell phones, I wonder if heterodyning might permit the use of these devices to record higher frequency calls. Introduce a 20 KHz local oscillator and a even a small device with a 4 KHz bandwidth may record information in the 20 KHz frequency range (albeit with some post-processing required). Realistically though, I suspect that while this is technically feasible it's enough of a PITA that folks would do better just coughing up the money for a proper higher bandwidth recorder - much like the case in macro photography in which extension tubes are certainly usable but most folks are ultimately better served by the purchase of a dedicated macro lens (hey look, we're talking about photography again...)
Heterodyning (had to look that up!) might be able to increase the frequency range of the input device (microphone), but if the recording device (phone) is using a compression algorithm which reduces the final recording to 4 kHz, it wouldn't matter what the external device was feeding into it.

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Re: Choosing a Field Recorder for Anurans - A Primer

Post by kricket »

I looked into all the above when I was trying to decide the best way to keep a record of the calls for the FrogWatch monitoring that I do, but even the least expensive of the good recorders was $200 and I've got no budget. So I just use what I have - my iPhone (which was a hand-me-down so it was free). And it works great unless it's really windy. I record with the voice recorder. It makes recordings good enough to identify all the calls, even in a messy chorus.


Here's one from earlier this year (I was right next to green treefrogs, green frogs and bullfrogs were farther away and Cope's gray treefrogs were in the trees quite far, but can still be heard. And there's a bunch of night calling insects. Plus there's a plane at the end since the site is on a flight path for National Airport!)

Not to diminish all the good advice you posted, but for those of us who don't have a lot of money (most of us?) and aren't planning on trying to sell our recordings then I just wanted to say you can still get decent recordings without fancy equipment. I just don't want anyone to be discouraged. :) Kara

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chrish
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Re: Choosing a Field Recorder for Anurans - A Primer

Post by chrish »

kricket wrote:I looked into all the above when I was trying to decide the best way to keep a record of the calls for the FrogWatch monitoring that I do, but even the least expensive of the good recorders was $200 and I've got no budget. So I just use what I have - my iPhone (which was a hand-me-down so it was free). And it works great unless it's really windy. I record with the voice recorder. It makes recordings good enough to identify all the calls, even in a messy chorus.

Not to diminish all the good advice you posted, but for those of us who don't have a lot of money (most of us?) and aren't planning on trying to sell our recordings then I just wanted to say you can still get decent recordings without fancy equipment. I just don't want anyone to be discouraged. :) Kara
Absolutely. You can record anurans with much less expensive equipment, although a $600 iPhone doesn't really count as "less expensive" :lol: . You can buy a $25 digital voice recorder at WalMart which will do as good a job as an iPhone. You could also say that you don't need a camera to take photos of herps because your phone has a camera.

But you are correct, the important thing is that we document amphibian breeding whether as part of the HERP database, state/regional databases and atlases or the North American Amphibian Monitoring Program. If you hear a chorus of frogs calling somewhere, why not take the five minutes to record it and report that data. Whether you record it with a $2000 microphone into a $2000 recorder or just with your cell phone isn't the important thing. Documentation is what is important.
I've been herping long enough to observe the extirpation of species in areas and the introduction of non-natives. I wish I had been documenting this stuff more completely 30 years ago.

One thing I can suggest is that there are recording apps for the iphone and android that do a better job than the simple voice recorder apps. PCM recorder by Tascam is a good free app for both platforms if you are using your phone to record frog calls.

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