Audio recording in theory
Recording sound in theory
Contents
Equipment list
- Microphones, mic stand, windscreen..
- Cables
- Recorder and mixer
- Headphones and speakers
- Bag
- Batteries (rechargables? solar?)
- Power cords, electrical connectors and converters, surge protectors (know the electrical standards for your country!)
- Removable media (SD cards, minidiscs...)
- Backup solution
- Gift solution (how will you make copies for people?)
- Toolkit
- Laptop
General properties of equipment components
- Dynamic range: ratio between maximal and minimal signals (given noise)
- Frequency response: ratio of output power to input power across range of frequencies, with constant input power
- Noise
- Weight and size
- Durability
- Cost
Cables
- Electrical specifications
- Unbalanced line: noisy
- Two-wire cable: hot, and ground (shield)
- hot - ground = signal + noise
- Advantage: cheap, simple, fine for short distances
- Disadvantage: noise may be introduced via stray electromagnetic fields on long cables
- Balanced line: no noise!
- Three-wire cable: hot, cold, and ground (shield)
- Signal is encoded on both hot and cold
- hot - ground = signal/2 + noise
- cold - ground = - signal/2 + noise
- (hot - ground) - (cold - ground) = hot - cold = signal
- Take care: the same pin must be hot or cold, else signals may cancel!
- Understand the issue with a number game:
- A wants to tell C a number, X by whispering to B who tells C. But B will add a fixed number Y, unknown to C. C hears X+Y and therefore can't know the original number!
- Trick: B must add the same number to all whispered numbers.
- So A tells B the number X as well as its negative -X. C gets Y+X and Y-X, subtracts them, and divides by 2. Voila - X!
- Unbalanced line: noisy
- Connectors and connections
- Balanced lines are carried by XLR connectors (3 pin/wire)
- Unbalanced lines are carried by quarter inch, miniplug, or RCA connectors (2 contacts)
- Stereo versions add an extra signal, thus 5-6 wire (balanced) or 3-4 wire (unbalanced), depending on whether ground is repeated.
- Thus quarter inch: tip, ring, sleeve (ground)
- Be sure ground is connected to ground
Microphones
The microphone is a kind of transducer, converting pressure waves into electrical signals by means of a diaphragm that vibrates with incident sound waves, generating an electrical current. The output is usually a low voltage (thousandths of a volt), which must be boosted to line level (about 0.5 - 2 volts).
- Induction technology
- Condenser (capacitor) microphone (capacitance change)
- Advantage: stronger signal, better frequency response
- Disadvantage: requires power (battery or phantom), more delicate, poor choice for loud sound
- Dynamic microphone (electromagnetic induction, like a reversed speaker).
- Advantage: Simpler, sturdier, fewer moving parts, no power required, good for loud sounds
- Disadvantage: Tend not to have flat frequency response, weak signals
- Piezoelectric microphone (piezoelectric generation)
- Condenser (capacitor) microphone (capacitance change)
- Directional pattern
- Omnidirectional
- Cardiod (front and sides)
- Bidirectional
- Hypercardiod
- Shotgun (too conspicuous!)
- Variable (some microphones provide a switch, e.g. to match video zoom)
- Special types
- Lavelier mic
- Wireless mic
- Contact mic
- Stereo mic
- Connectors
- Impedance
- Low: under 600 ohms (most condenser mics are under 200 ohms)
- High: over 10k ohms
- Low impedance is preferred. High impedance will not perform well with longer cables.
- Connect mic to system of equal or higher impedance
- Frequency response
- Plot of dB vs Hertz (input signal), where dB measures the ratio of output to input
- Flat is generally best, but at least match to expected frequency range
- Condenser responses are generally flatter than dynamic
- Dynamic range
- essentially ratio between loudest sound to noise floor
- Accessories
- Windscreen
- Mic mount (reduces vibration)
- Stand
Digital recording
Recorder type
- Computer to HD
- Dedicated hard drive recorder
- Flash memory (SD cards, etc.) recorder
- Minidisc, Hi Minidisc
Digitization
- AD converter: quality important (use outboard converter)
- Sampling rate
- Sampling rate should be greater than twice the highest frequency
- Human hearing is limited to about 20kHz
- So sampling rate should exceed 40 kHz. Typical rates:
- 44.1 KHz (used for CDs)
- 48 KHz
- 96 KHz
- 192 KHz
- Bit depth
- More bits increases dynamic range, decreases quantization noise, but increases file size
- 8 bits (49.93 dB SQNR)
- 16 bits (98.09 dB) (used for CDs)
- 20 bits (122.17 dB)
Setting levels
- Automatic level control
- Limiter
- Manual level control
- Danger: clipping!
Uncompressed audio
Use Pulse Code Modulation (PCM) to encode the analog signal
- wav (pc)
- aiff (mac)
- bwav (incorporates metadata)
Lossless compressed audio
- flac
Lossy compressed audio
- mp3
- ogg
- mp4
- wma
Considerations when designing a recording system
- Cost
- Cost of equipment
- Cost of repairs
- Cost of supplies, per minute of recording
- Cost of power, per minute (batteries)
- Quality (Signal/noise, distortion)
- S/N ratio
- Dynamic range
- Frequency response
- Other sources of distortion
- Durability
- Durability of equipment (given environmental problems: heat, dust, moisture, light, shock, etc.)
- Stability of the medium, durability of the recording itself
- Feasibility (practicality)
- Portability of recorder (weight & size)
- Compatibility of equipment types (e.g. computer and recorder)
- Conspicuousness (small is less, but may make less good recordings), e.g. shotgun mic may be technically useful, socially useless
- Practicality of supplies (availability)
- Practicality of power source (availability, voltage, etc.)
- Ease of working with the medium (copying, transferring, editing, etc.)
- Amount of storage (tape, HD, etc.) required (tape length, total volume/weight of tape required)
- Duplication ease and generational loss
- Able to handle range of situations (interview, music, inside, outside, etc.)
- Ability to combine video recording a plus!
