At Somerdata, we have selected for you the pick of the best around, and where we don’t carry them ourselves, we will recommend the best solution for you.

Unlike Video recording, audio can be quite difficult to understand and there are still a plethora of formats, rates, technologies and so on to choose from.

So, first try and define what you want to achieve, then compromise to what you can get ( and afford!).

Here are some questions to pose, only you can supply the answers.

1. Is my target benign or hostile? How will they react to discovery of their being recorded? Do I need to tell them beforehand? ( you might need to do some research on lawful intercept and fair usage in the territory in which you operate.)

2. Is my target stationary or mobile? Do they visit one location or many?

3. How close can I get?

4. Can I get to the target site(s) safely before  and /or after the surveillance period?

5. How long is the surveillance period? Is surveillance unattended? Does it need immediate reaction to events?

6. Do my recordings need to be presented as reviewable evidence, especially in a court of law.?

7. Does my device need to be self-powered? How physically robust does it need to be? Are there size and packaging constraints?

8. Does the device need to be protected from counter-measures and interception? what happens if recordings fall into the wrong hands or transmissions are intercepted by a third party?

9. How much is my data worth? How much am I prepared to spend to collect it?

If you’ve answered ALL of those questions then you probably already know enough to make your own selection and you don’t need any more advice. Carry on!

For everyone else,  let’s look at the options, technologies and practical solutions.

Firstly, if you carry a mobile phone, then you already have a mobile audio recorder. Put it on the desk, point it at your target and press record. The quality will be ok and you’ll get something. Now here’s the weird thing. When you come to play it back, you’ll suddenly notice all sorts of strange noises you didn’t hear when you made the recording. Rustling paper, room echo, road noise, and a hundred other liltte noises, some quite distracting. With luck you may even have recorded something useful. Why is this?

Well, the human auditory system is a remarkable instrument. It can filter out extraneous noise, correlate muti-path echoes, focus on point sources and acts as a very sophisticated filtering system especially when coupled with eyesight. Human ears have a very wide dynamic range (>120dB) meaning we can hear whispers and jet engines with equal clarity, but we also non-linearly favour speech frequencies – we hear in the range 300 to 6 kHz more distinctly than in frequencies outside this range. And this also varies with age.

Crucially though, we hear in Stereo using two ears. This not only gives us twice the sensitivity but we are able to give spatial resolution to what we hear – where it is, how fast it’s moving, separation of multiple sources into distinct focal points.

So technically, the mobile phone is going to struggle a bit to compete. It’s  probably mono, limited dynamic range, limited frequency range and doesn’t match the human ear response.

Which leads us to the grown-ups – dedicated recorders and transmission systems designed specifically to work in this environment.

First Recorders.

Dedicated audio recorders for use in covert and /or surveillance operations generally  need to be physically small, robust, high quality and easy to operate. In addition, they may need to have long recording time, be secure if mislaid, and invisible to counter-measures.  Some if not all of these are contradictory and require practical compromises.

Lets look at some of these.

Quality – by which we mean intelligibility and repeatability.

We need to be able to understand the recording and probably listen to it many times without degradation. Repeatability comes from digitising the audio, then using a high-quality medium to store the resultant digits. Nowadays this means solid-state (semiconductor) . Not infallible, but better than mechanical systems like tape, disk or cd/dvd which are easliy damaged.

Intelligibilty comes about through a combination of dynamic range ( the ratio of the loudest sound to the quietest) and frequency range ( bass to treble).

Digitisation involves cutting your nice analogue signal into thousands of bits that take up a lot more space and take longer to transmit than the original, so the concept of compression was introduced to compenaste.  This is not the place to debate compression algorthms, suffice it to say that compression, especially non-reversible, is generally bad for intelligibility. Digitisation also creates its own limits. As a rule of thumb,  you need to digitise at twice the highest frequency you want to record at – as a practical  example, 8kHz of voice bandwidth needs to be digitised at 16KHz. ( For comparison, audio CD quality  digitises at about 44kHz to get a HiFi acceptable bandwidth of 20kHz).  Note though that the number of bits created also doubles, so does the storage capacity and the transmission rate.  In general you need the highest sampling rate you can afford ( although there is no point in outsripping the capability of you microphone). And from an intelligence gathering point of view, you may want to include a wider bandwidth than is strictly necessary for voice, to improve spatial information or pick up extraneous sound information that gives context to the operation.

The other determinant of quality is the number of bits per sample, the quantization level. 8, 10, 12,14 and 16 bits are common. The practical effect of quantization is to largely determine the noise floor, or put another way, the overall signal-to-noise ratio and thus the dynamic range. The more bits per sample  used, the lower the noise floor and the wider the dynamic range; so 10 is good, 16 is better. Translated to analogue quantities, 10 bits is about 60dB ,  16 bits can represent about 96dB of Signal to Noise ratio. Of course this is not the only determinant of system noise -  power supplies, microphone and pre-amplifier semiconductor noise may combine to dominate. And as usual there is a price to pay for more bits, in power consumption, download/transmission time  and storage space required

Discrete – or in other words very small and unobtrusive, preferably invisible. Packing a lot of data into a very small space is a challenge for both microelectronics and power supplies. In fact, portable device size is  almost always determined by the battery size which in turn is determined by the power consumption and operating time of the electronics -which in turn is determined by sampling rate and storage technology…… etc.!

Invisible is still a bit tricky but consider hiding in plain view – a pen, a key fob, a usb stick or a bluetooth earpiece are all available and ignored by most people.

Secure – faced with the inevitability of losing your recorder at some point, what about its content? Would you want that falling into the wrong hands ? (ie not yours).  Securing the contents of the recorder is vital, so consider using a record-only device. This may sound strange bt what it really means is beign able to retireve the data only under controlled conditons. A password protected recorder/replayer is a good start but most passwords can be cracked quite readily because they are created by people who liketo be able to remember them so tend to submit to computerised brute force attack quite readily.

Encrypting the stored data is another help, now you need a password ( you did set one didn’t you….!)  and a key. But encryption also takes up storage space, consumes power and is not readily available on simple devices.  Storing the data in an unfamiliar format, proprietary or obscure is another method. users now need access to a format converter which may be unpublished or simply unavailable. This is almost as effective as encryption and effectively limits access at no overhead cost to the device.

Alternatives to Recorders

The obvious alternative to recorders is direct transmission and the uninitiated often say, “all i need is a microphone with a transmitter and i can use wireless – i don’t need a recorder near my target”. Let’s examine that in the same technical way as we did above.

Wireless transmitters suffer many of the same constraints as recorders with a few added. Firstly, a microphone or audio gathering system still needs to be in the right place. It needs a power supply, a transmitter and an antenna. If it needs to be externally controlled it needs a receiver as well.  Wireless transmissions are subject to strict regulation – you can’t just use any part of the wireless spectrum for your transceiver. In fact there are very few parts of the spectrum available to non-broadcast or military devices and they come in distinct bands, for ecample 900MHz, 1.8GHz, 2.5Ghz, 433MHz, 27Mhz and so on. Why is this important? Firstly, these bands are already heavily used -cellular phone networks, bluetooth, zigbee, low power radio, radio controlled models and so on. Secondly and more importantly, their transmissibility varies with frequency or wavelength if you prefer. Short wavelengths ( higher frequencies) travel in straighter paths, the so-called line of sight problem. Short wavelenght radio  doesn’t travel round corners. So shaped antennae are required that ‘point’ signals in the desired direction. Perversely, for this argment, the more useful longer wavelengths that can travel further and  bend a bit, need physically larger antennae. In the case of the longer wavelengths, a quarter wave antenna can be half a metre long. Go lower in frequency and we start to talk several metres (or several equivalent metres) with folded arrays and reflectors etc. These can hardly be described as discrete.

So surveillance trnasmitters of this type tend to concentrate on the shorter wavelengths. For example Bluetooth operates in the 2.5GHz region. Consequently the range is limited and if someone happens to park a bus or a truck in front your transmitter, or it rains heavily, or there are a lot of other bluetooth devices around then the range may be zero as far as you are concerned!.

The effects of this can be overcome a little by boosting the power of the transmitter, bt leaving aside the increased power supply or batter needed to do this, this leads to the next problem -detection and counter-measures.

Clearly, a wireless transmitter by design is capable of being found by a receiver. If it can be found by your receiver then it can be found by someone else’s. Or more likely by a sweeping spectrum analyser designed to find it.  Strategies to overcome this include spread-spectrum, chirp or burst transmission and sweep detector detection. These add to the complexity, cost and detract from the reliability.

So what about a hybrid? Why not record the data, then download it wirelessly so you don’t have to retrieve it.?

In two words -see above. The problem does not go away or get better. The wireless link now has to be remotely activated, so a transmitter and receiver is required. And an antenna. And a power supply. And a recorder. And let’s think about the data transmission characteristics.

Suppose a recorder has been running for a couple of days at a good quality. It may have accumulated 3-400MBytes of data. This now has to be downloaded. Assume the link is capable of good quality bluetooth  rates, say 100kbps to make the sums easy. The time to download this data assuming uninterrupted and error-free, is 400*8*1000 kb (kilobits) /100 (kilobits per sec) = 32000 seconds or about 9 hours!

Now clearly we can hope for better than this – low power data rates are gradually climbing but even at 1Mbps this is still approximately 1 hour. Higher data rates approaching WiFi rates are available but only at significant power cost. And of course the transmitter can still be detected, and any simple electromagnetic shielding around the target will nullify the transmistter. (Chicken wire works well).

The conclusion to this is that there is no simple answer. Each situation may require a different solution and you need a toolbox of them to deliver results.

Hopefully at Somerdata we can provide you with some of these tools, advice and even the names of Companies  who provide the solutions we don’t. !

Model SSAS-A37 A37 Data sheet comes in 2 or 8GByte capacity and features a small size with a high capacity internal rechargeable battery for up to 40 hours continuous high quality recording.

SSAS-A37

Model SSAS-A9 Plus SSAS-A9 Data sheet has even longer single charge recording time, up to 210 hours and also has an integrated fast USB 2.0 interface for speedy downloads and local replay for review monitoring.

SSAS-A9

Model SSAS-U49 SSAS-U49 Data sheet is a unique directional recorder that includes multi-microphone noise cancelling technology for unbeatable performance including a recording  range up to 15 m from the target!.  Noise cancelling also contributes to the very wide dynamic range of this recorder, up to 80 dBm.

SSAS-U49

SS-ASR-S Miniature Audio Surveillance Recorder Module

The SS-ASR-S Stereo Audio Surveillance Modules have very small dimensions and weight, long recording times, very low power consumption, wide frequency range (100Hz – 10,000Hz), wide dynamic range and a highly sensitive external microphone.

With the right technical capabilities, the module can be integrated into a variety of everyday objects for covert deployment.

The absence of moving parts means the recorder can function in a wide temperature range, under vibration and in dusty conditions.

It is possible to record by both daily timer and one-off timer.

Recordings are stored in a  proprietary format to prevent unauthorised access and retrieved and managed by attaching the unit to a PC via USB and running the supplied software.  Recordings are then available as   date/time-stamped WAV files.

Data files are signed and can be checked for auditing and anti-tamper purposes.

To secure recorded data, a password may be set which prevents another user from unauthorised access to content and settings.

With up to 24GB flash memory, giving over 5000 hours of continuous recording, the SS-ASR-S Module is one of the most compact and successful recording modules for covert operations.

A range of accessories is available including miniature battery packs, high sensitivity miniature microphones, connector options and wired remote switching.

See also our  Development Kits for a quick way to customise your design for special applications.

Accessories

• External sub-miniature regulator
• High capacity miniature primary power source
• sub-miniature high-sensitivity microphone
• remote switch and indicator

Dimensions

• SS-ASR-S1: 31mm x 25mm x 6mm
• SS-ASR-S3: 45mm x 30mm x 6mm

SSAS Audio Modules Data sheet (pdf)

SSAS Audio Module Record Times (pdf)

SS-ASR-B22 Miniature Audio Surveillance Recorder

The SS-ASR-B22 is a low power, high capacity covert digital recorder.

The SS-ASR-B22 combines small size with large recording capacity; it’s the size of a large postage stamp, and is available with up to 8GB of flash memory, giving a capacious 1706 hours upper limit to recording time.

It possesses a built-in high-sensitivity microphone and battery holder. This means no external cables or microphones are required, providing enhanced concealability.
Power is from a quick change standard button-cell.

Recording may be controlled using a simple slide-switch on the side of the unit. Four recording modes are provided: Single–Pass recording, Endless-Loop recording, Voice Activation and Scheduled Timer mode.

Recordings are stored as date/time-stamped WAV files with 10-bit A/D resolution. Recordings are retrieved and managed by attaching the unit to a PC via USB and running the supplied software.

Data files are signed and can be checked for auditing and anti-tamper purposes.

With the features detailed above, we believe the SS-ASR-B22 to be one of the most compact and effective recorders available!

SS-ASR B22 Data sheet (pdf)

SSAS B22 Recording Times (pdf)

New!!! The LCD SSAS-A10 Model

If you need long continuous recording of voice messages,  with display confirmation, the SSAS LCD A10 is the recorder you need.

With up to 2400 hours of built-in memory capacity, it can record 350 hours before needing re-charging. Available in 2, 4, 8 and 16GByte capacities for long-duration recording.

This recorder is characterized by high recording quality as well as low power consumption and as in all recorders of the LCD series there is an option to replay the recordings through earphones.

Distinctive features:

• Light aluminum case;
• LCD indicator – 22,2×12,9 mm;
• Joystick control;
• Dimensions 51x50x13 mm;
• Weight 47 g ;
• Power supply: Li-Pol rechargeable battery
• Rechargeable battery capacity: 950 mAh;
• Battery life in record mode up to 350 hours;
• Built-in flash memory capacity: 300, 600, 1200, 2400 hours;
• VAS system to compress pauses when recording;
• Built-in real time clock;
• Timers for automatic scheduled recording;
• Information protection system.

Technical characteristics:

• Battery life in stand-by mode: 2 years;
• Frequency band: 100÷6000 Hz;
• Signal-to-noise ratio: –72 dB;
• Built-in microphone sensitivity: 7÷9 m;
• Bitrate: 1,5 Mb/s;
• Charging time: 2÷3 hours;

A10 LCD Data Sheet

A 100 day recorder! – The new A9 consumes less current  than its predecessors, providing longer autonomous operation between charges. The recorder is housed  in a durable metal case and can be easily be concealed on a key-ring.

Now you can continuously record for up to 210 hours (more than 8 days) or 100 days in voice activation mode . You can even  listen to the recording using  headphones plugged into the recorder.

Measuring just 11*27*56mm and available in 2GByte to 8GByte formats, the Lithium Polymer battery delivers exemplary service life.

o Enhanced management and debug software
o Comprehensive instructions and examples
o Audio editing software (3rd Party)
o Spice modeling software (3rd Party)
Please note – all 3rd party software and links to trial software are provided as a courtesy without warranty or support from Somerdata.    Individual licence conditions apply in all cases.

SSAS Module Data sheet (pdf)

SS-ASR-A31 Audio Surveillance Recorder

Guinness World Record Winner!

The SS-ASR-A31 is the smallest recorder in the world, being 12% smaller than its predecessor the SS-ASR-B21

2GByte – 8GByte flash memory capacity, giving up to 1200 hours continuous recording time

Built-in internal high-sensitivity microphone & rechargeable battery giving 25 hours of continuous recording – no external components. This makes this Recorder even easier to conceal and deploy

Metal casing & solid-state internal components provides a rugged product ideal for harsh environments

It is possible to record by both daily timer and one-off timer.

Recordings are stored as date/time-stamped WAV files, and are retrieved and managed by attaching the unit to a PC via USB and running the supplied software.

To secure recorded data, a password may be set which prevents another user from unauthorised access to content and settings.

Data files are signed and can be checked for auditing and anti-tamper purposes.

A31 Data sheet (pdf)

SS-ASR-B8 Audio Surveillance Recorder

The SS-ASR-B8 is a high capacity covert digital recorder with low power consumption, LCD display, voice activation, integrated timer and selectable compression option

The SS-ASR B8 allows the user to replay recordings through the Recorder itself and is not limited to only replaying through a computer

2GByte – 16GByte flash memory capacity, giving up to 4778 hours continuous recording time

Built-in internal high-sensitivity microphone & quick-change coin-cell battery – no external components

Easy-to-use joystick and LCD screen for control and status.

It is possible to record by both daily timer and one-off timer.

Recordings are stored as date/time-stamped WAV files, and are retrieved and managed by attaching the unit to a PC via USB and running the supplied software.

To secure recorded data, a password may be set which prevents another user from unauthorised access to content and settings.

Also available is a removeable media version using a mini-SD card to store the data.

B8 Data sheet (pdf)

SS-ASR B8 Recording Times

Audio Surveillance Short Form (pdf)