Electronic sound issue in Round 5 device

Thank you very much for this information, I will change the batteries before using them again this month. Is there anything I can do to avoid the noise due to the temperature?

Sampling rate 384kHz, and the card is SanDisk Extreme 32GB (not extreme pro) the audio moth devices are in a forest (inside a ziplock), an yes, it ocurres in some recordings. Also, sometimes this noise appears too at 86kHz:

There is also some small background noise from the SD card writes but this seems to be more pronounced. What sampling rate are you using? Is this an Extreme or Extreme Pro SD card? Is the AudioMoth in a housing? Does is always occur or just on some recordings?

Hi, I am having the same problem with the "clicking" noise at 18-20kHz. If I slow down the rate to 0.2 it sounds like a clock. I have 4 AudioMoth devices, all with Energizer AA batteries and sand disk extreme of 32gb. The noise appears in a few recordings regardless the device (it appears in all of them but we don't know what's activating it). However, when analyzing the spectrogram it becomes an issue because it overlaps with the sounds we are interested in.

I would try a different SD card first if you can - I don't think we've ever tried a Sandisk Ultra card. Also using batteries is also safer as it isn't clear how noisy the USB power supply will to be under varying load - some might work better than others. All our recordings are on Sandisk Extreme and either regular Alkaline AA or Energizer L91 AA (http://data.energizer.com/pdfs/l91.pdf) batteries.

Can you do a test recording inside with the device about 50cm away and just count out loud at normal speaking volume, with the device recording at 48kHz and gain level 2. Preferably use standard Alkaline AA batteries for this. The recording should be nearly noise free with very clear audio. If you email me the recording (alex.rogers@cs.ox.ac.uk) I can check it.

Hi Alex

I put the unit out for a couple of nights using the 5V usb - the same SD card - but I think the results are worse. There is a very audible beat.

The bottom axis is seconds.

Regards

Ian

HI Alex

Thank you for getting back to me.

You can hear the click when you reduce the speed (so that you can 'hear' bats).

The batteries are Energizer NiMh fully charged (4.2V) and the card is a Sandisk 32G Ultra.

The batteries lasted for over 4 nights, in fact they lasted longer than the card capacity.

Can you connect a 5V USB supply? If so what switch setting do you use?

Regards

Ian

I have noticed that there is a 'clicking' and occasional 'buz' at around 18KHz - 24KHz on all of my recordings.

Recording set to 48K, 24bit and this is a snip.

Regards

Ian

I've been using Eneloop LSD NiMH rechargeables without problem. The voltage may be only just above what is required but it remains very constant. Apart from having to buy and dispose of many primary cells I can always put a set in the device knowing it is fully charged. If primary cells are used their capacity will be unknown unless a new set is used for every session. Will the Audiomoth 2 voltage booster work from 2.4V?

The AudioMoth uses a voltage regulator to generate the 3.3V power and signals required by the SD card. This regulator has a dropout voltage of between 0.2 and 0.3V depending on load which means that the battery voltage needs to be roughly 3.5 to 3.6V to maintain a regulated 3.3V supply. Everything will probably continue to work fine below this threshold (the microphone and processor will continue until very low voltages) but the voltage is going out of spec for the SD card.

With Energizer Lithium cells you get pretty much the full capacity of the cell as they discharge from 1.65V down to 1.2V. Alkaline batteries aren't quite so good as the 1.2V threshold is reached earlier on the discharge curve. These were the batteries we planned to use so optimised the number of cells to match.

With Ni-MH batteries, and Ni-Cd, you are pretty much at the threshold from the very start due to the lower cell voltage so it's not so clear how much of the stated capacity you'll actually be able to use. At some point the SD card will start to complain about the low voltage and this probably depends on the make and type of the card. You could certainly use four cells, and large 4.8V Ni-MH or Ni-Cd battery packs would work well.

On AudioMoth 2 we're going to use two AA batteries and a 3.3V voltage booster IC to increase the battery voltage up to 3.3V. This is a bit less efficient than a voltage regulator but you win that loss back by being able to drain the batteries completely and access their full capacity. It also makes the device a bit smaller and cheaper to deploy. The newer processor also uses significantly less power.

Alex

Please could you clarify your comments about NiMH cells. You first stated that the cell voltage is too low for the Audio Moth then went on to say that you are reducing the number of cells from 3 to 2 - that just does not make sense. If you are running so close to a voltage cut-off then reducing the available EMF by one third would not seem to be a very sensible idea! What is the cut-off voltage for operation of the AudioMoth?

Regarding EMF of the two chemistries - the voltage of a NiMH cell remains fairly constant at around 1.2 volts for most of its discharge cycle whereas that of an Alkaline manganese dioxide cell slopes steeply and falls rapidly to around 0.8 volts by the time it is 95% discharged. In addition the internal resistance of NiMH cells is very much lower than alkaline so less of a drop in voltage under load. The two factors together mean that in general an NiMH cell will, in many applications, actually continue to supply a greater EMF than an Alkaline of comparable capacity for much longer despite its considerably smaller nominal cell voltage.

Experience seems to show that the AudiMoth does appear to function well using NiMH cells - at least with the current complement of 3 cells.

These two events occur about 45 seconds apart and look pretty uniform.

Could be a phone interfering, or a wireless router, if nearby?

The battery cases are quite low quality and some have bends in them. We've revised the design in the next version which will have just two AA batteries is separate better quality holders. This should also help with manufacturing where we'll have a better process in place to align the mounts and the PCB. This version proved much more popular than we expected.

The problem with the audio seems to be a strong 20Hz signal creating interference.

This is strange since nothing on the board is operating at that frequency. In particular, SD card writes occur every 0.34 seconds and that is the most likely cause of any interference if there is some resistance in the power circuit. The interference does look like a short step change in the input signal to the ADC which the software DC filter then turns into the two short ramps.

Can you try making a recording outside away from any obvious source of interference such as a laptop, phones or artificial lighting. We've had USB power supplies and fluorescent lighting generate interference in the past, but I'm not sure we've seen it at this low frequency before.

You probably won’t get much life out of NI-MH batteries since you the voltage is close to the lower limit even when fully charged.

Is the card new? If not try reformatt it as a FAT drive before using it.

Can you send me a WAV file to look at by email.

The cell voltage of rechargeable NI-MH cells is too low. Try standard 1.5V Alkaline batteries.

Are you running from batteries or from a USB power supply?

I'm using a SanDisk Extreme 32GB microSDHC UHS-3 Card with the following settings.

Hi, Can you confirm what recording settings you are using and the exact make and model of the SD card? Are you running from batteries or USB power? Alex