The Trials & Tribulations of Field Recording – A Beginner’s Guide Part III

 

‘Watch out – moketo…moketo…. you’ll ged bid’   

[65-year-old man]   ‘What did you say?’  

[8-year-old boy] ‘Are you deaf?   ‘I SAID, watch out – mosquitos – mosquitos - you’ll get bit’

[65-year-old man] ‘Ok no need to shout – I can hear you  -  cheeky little bugger’

[25-year-old man] ‘No you didn’t – you said mosquito, mosquito, you’ll get hit -  so just clear-off and play somewhere else’

[8-year-old boy as he runs off laughing] ‘You’re both deaf ’

[65-year-old to 25-year-old men both scratching their heads and waving their arms around] ‘ These damn mosquitos bite hard – I’ve never seen so many’

 

This all leads nicely into the next subject……….

Recording Formats:

 There seems to be an ever-increasing need to record at higher and higher sample rates and bit depths, a trend fuelled by audio equipment sales hype, which is then taken up by the many Hi-Fi buffs who expound their theories on the many audio forums about the pros & cons of purchasing the latest equipment to hit the market……. it’s a form of anti-ageing cream for the audiophile.

You may have already picked up on my repeated reference to ageing, a fact of life that can be ignored but unfortunately not avoided – even by the aforementioned self-proclaimed experts. If you watched the video above, you may now be aware of your own hearing limitations – but don’t be disheartened by your findings as it’s a fact of life – we all succumb to it sooner or later!

The acknowledged range of human hearing is 20Hz – 20,000Hz, but, and this is a big BUT, it reduces quite dramatically as a result of age-related hearing loss, which can start anywhere between the ages of 8 and 20 years. For instance, a 65-year-old male tends to have a hearing loss of 77dB at the reference frequency of 6000Hz and for the same frequency this loss is 25dB for a 25-year-old male. These losses mentioned are considered as being just within acceptable audiometry limits.

In general (I’m not interested in whether you can or not, or what the highest frequency is that you can hear) the upper limits of hearing for different ages groups are as follows:

Age                            Upper Frequency Limit (kHz)

Under 24 years                 20

Under 30 years                 16

Under 40 years                 15

Under 45 years                 14

Under 50 years                 12

Under 60 years                 10

Above 60 years                7-8

We’ll return to this later.

Advances in digital recording technology has brought with it a large range of recording modes available for use in digital audio recorders; unfortunately this large choice can be overwhelming for the novice and may result in an inappropriate format and mode being chosen. In order to avoid this, a basic understanding of these figures specified for your particular recorder is essential, prior to getting out in the field and pressing that ‘record’ button.

As field recordists, we must keep reminding ourselves that we are operating at the front-end of the recording chain. We are responsible for capturing those analogue sound-pressure waves, by means of our microphones, which then convert them into an electrical alternating current which flows down the cabling to the recorder. The recorder then amplifies this weak electrical signal and then converts it into a digital signal by means of an analogue – digital converter (ADC). The digital signal may then be further processed if required, or merely stored in cells on a flash memory card.

Our choice of recording mode and format basically controls how the ADC is to analyse and convert the analogue electrical signal sent by the microphone  – it’s a very important decision to make, as it not only affects the basic building blocks of the final digitized signal, but also how it will be stored and subsequently accessed.

Here is a typical manufacturer’s specification  showing the recording modes and formats available for a modern portable recorder; which includes three of the most common recording formats currently in use. There are other less common formats to be found, but these tend to be manufactures’ proprietary ones for use with their particular equipment; to simplify matters we will stick with the commonly used ones shown here.

Recording Modes & Duration for an 8 GB Memory Card:

WAV (PCM) Format
96kHz / 24bit                3 h 50 min
96kHz / 16bit                 5 h 50 min
88.2kHz / 24bit             4 h 10 min
88.2kHz / 16bit             6 h 20 min
48kHz / 24bit                7 h 45 min
48kHz / 16bit                11 h 45 min
44.1kHz / 24bit             8 h 30 min
44.1kHz / 16bit            12 h 45 min
Mono                              25 h 40 min

MP3 format
320 kbps                        56 h 50 min
256 kbps                         71 h 05 min
128 kbps                         142 h

WMA format
160kbps                          111 h
64 kbps                          278 h

Only one of these may be considered as an uncompressed digital file, that is the WAV (PCM) format. All others are subject to processing by means of compression algorithms which basically reduce the size of the audio file by stripping out non-essential audio or bits of data. The benefits of compression are two-fold, that of being able to use smaller recorder memory cards with lower data transfer rates and the ability of the recorder to operate for longer periods and with lower power consumption. Unfortunately these benefits are offset by the reduction in audio quality, as these compression algorithms strip out sections of audio frequency considered to be non-essential for the average listener.

The WAV (PCM) format:

The WAV format is what is termed a container format, which can hold compressed file formats, but normally holds uncompressed linear pulse code modulation (LPCM normally identified as PCM) data. This format also holds additional information enabling cross-platform use allowing these files to be played on most audio players. The different modes available with this format are identified by 2-components, its sample rate and bit depth.

Let’s consider the standard audio cd WAV format of 44.1kHz/16bit. The 44.1kHz represents the sample rate at which the audio data is digitized, in other words there are 44,100 samples taken every second (44.1kHz) and the audio volume for each of those samples is then measured. How finely the signal volume can be measured or resolved for each sample depends on the bit depth available to store the information. In our example we have used a bit depth of 16, so the volume can measured and stored in the audio file as 65,536 separate bits of information. If we had instead chosen to use an 8bit WAV format then there would only have been 256 separate bits of volume information available for storage; in other words the resolution of the signal would have been much lower compared to 16bit recording.

Without resorting to complex mathematics, the sample rate of 44.1kHz immediately tells us that the maximum recordable frequency for that particular mode is about 22kHz (simply halve the sample rate) and being 16bit it will be able to record a dynamic range of 96dB (simply multiply its bit depth x 6).

Let’s repeat this simple exercise for the highest sample rate and bit depth mode, comparable to DVD audio quality (WAV format: 96kHz/24bit) specified for our recorder above:

The sample rate  of 96kHz  will enable us to record up to a maximum frequency of 96kHz x 1/2 = 48kHz

 The bit depth of 24bit will enable us to record with a maximum dynamic range of 24bit x 6 = 144dB

For those unaware of the term ‘dynamic range’, expressed in decibels(dB); it is simply the difference between the loudest and quietest sound pressure levels (dB SPL). For human hearing the dynamic range is 120dB, where 0dB is the quietest sound (threshold of hearing) and 120dB the loudest ( threshold of discomfort); but consider this, 130dB is the threshold of pain.

So looking again at the results of our 96kHz/24bit recording mode – we’re obviously pushing the limits here by recording at a frequency which is more than double the frequency heard by those youngsters with excellent hearing and probably six times higher than that which most people can hear! ………..and to top it all with a dynamic range which can inflict severe pain and damage to the human body.

As a comparison, take a check of the dynamic range of your own audio system, you’ll be surprised how low down it comes in this order of dynamic range.

http://www.audiocheck.net/audiotests_dynamiccheck.php

Apart from the obvious implications stated above, recording at sample rates and bit depths which are unnecessarily high can have a profound affect on subsequent archival file storage and in some cases the inability of other hardware and software being able to read or process the resulting audio files. Although not shown for our particular recorder, some recorders are capable of recording in a WAV format of 192kHz/24bit. This high rate can be problematic when recording direct to compact flash card as few cards can sustain the necessary write speeds for reliable recording at that rate.  

The following illustrates the large difference in file storage requirements between a few of these recording modes for a typical 30 minute recording:

Approx.
File Size (MB)                               Recording Mode &  Format

1000                                                  96kHz/24bit stereo WAV(PCM)

300                                                   44.1kHz/16bit stereo WAV (PCM)

75                                                      WMA, VBR Quality 100, 44 kHz, 2 channel 16 bit

28                                                     MP3, 44.1kHz, 128kbps Stereo

3                                                        11.5kHz, 16kbps Mono

The other two recording formats mentioned, the MP3 and WMA are both compressed audio file formats and have recently seen an upsurge in use with the introduction of personal digital audio players with access to online music stores.

The MP3 (MPEG-1 Audio Layer 3) format:

With this format, the audio file is compressed down to approximately 1/12 that of a WAV file, making them much smaller, economical to store and easy to transfer online; hence its popularity for online music distribution. The downside of these compressed files is the poor audio quality compared to uncompressed WAV files.

The WMA (Windows Media Audio) format:

This is a proprietary Microsoft compressed audio file format  which can support digital rights management for copyright protection. Although this format uses even higher compression ratios than the MP3 format, it supposedly yields better audio quality than the MP3. Its use is gaining ground over the MP3, especially for small personal digital audio players, where memory is at a premium and there is a need to store a library of music files. The WMA format is more advantageous to use than the MP3 format when used at the lower bit rates such as 64kbps.

The different modes of MP3 and WMA compressed recording formats are identified by their data transfer rates known as ‘bit rate’, in kilobytes per second (kbps), rather than sample rate and bit depth as with the uncompressed WAV format. The larger the compression – the smaller the bit rate.

Hence it will be seen from the above manufacturer’s specification that the 128kbps MP3 mode will be twice as compressed as the 256kbps MP3 mode. Although as just stated, the higher the compression – the lower the bitrate, we can also add, the lower the audio quality, but this only applies when comparing like for like formats; we cannot assume this applies when comparing different formats. It may well be that a more compressed 64kbps WMA has a higher audio quality than a less compressed 128kbps MP3 format – it tends to be subjective due to the psychoacoustic principles used in the compression codecs.

So now we know some of the basics behind the commonly used recording modes and formats, how do we make the choice of which to use?

The key to this decision is by asking yourself a few simple question: -

1. Will the audio file be archived for prosterity?

         Yes – then go to 4             No - then go to 2

2. Are you recording for high quality audio?

         Yes – then go to 4              No – then go to 3

3. Will the recording be copied for conversion to other file formats?

         Yes – then go to 4             No – then go to 5

4. Use WAV (PCM) format

If there are no specific recording requirements then make your own choice based on file storage requirements. If you know beforehand the frequency and dynamic range likely to be encountered whilst out recording, then use the simple rules of thumb mentioned above to work out the best WAV mode to cover these. Don’t get carried away by using unnecessarily high sample rates or bit depths. CD Quality WAV(PCM) 44.1kHz 16bit Stereo is a useful audio benchmark to remember, as it encompasses the full range of human hearing with respect to both frequency and dynamic range.

5. Ideal for use where file storage and transfer speeds are more important than high audio quality. The choice between the various MP3 & WMA recording modes will be influenced by the need to balance audio quality against file size.

The choice of format for use on personal players may be limited by its ability to recognize certain file formats, otherwise it’s purely subjective, based on personal preference. There are millions of music lovers around the world quite content listening to downloadable music with bit rates of either 128kbps or 192kbps in WMA or MP3 format.  At this point it might be appropriate to refer you back to the start of this post – how good is the YOUR hearing?

As a guide:

MP3 128kbps  or WMA 160kbps Stereo may be considered for higher quality internet audio.
MP3 56kbps or WMA 64kbps Stereo may be considered for medium quality internet audio, note taking etc.

If there are no important criteria to influence your decision, or you are simply uncertain, then play safe and record at the standard CD WAV(PCM) 44.1kHz/16bit format.

A word of warning: There are many good file conversion programs available capable of converting from any conceivable audio file format to another, but do not use these for converting between compressed file formats, as the resulting audio quality will be much lower than expected for the new format. If you need an audio file in some other format, then where possible always convert from an uncompressed WAV (PCM) file to the new format.

Best of luck and have fun!

 ……………

Beginner’s Guide Part I

Beginner’s Guide Part II

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