Chapter 3 Notes - Posted on 2017-01-12 by Nicholas Casas

Chapter 3 Notes

• Digital sound is an illusion. We are perceiving multitudes of discrete, instantaneous samples which sum up to a singular sound.
• Transducer: An object which converts one form of energy into another.
• Sample and hold: When a signal is amplified, a transducer will read the signal, then hold it until the next available one arrives. This saves energy and eliminates variations that can cause problems during the conversion process.
• The higher the sample rate, the clearer and more accurate the sound wave will appear.
• Computers use an INTERPOLATION ALGORITHM to smooth out the lines and make the waveform more accurate.
• Most people can't differ any audible differences between 44.1 kHz and 48 kHz. But there is a huge difference in accuracy between the two.
• ADC: Analogue to digital converter. These receive the sample and hold circuit voltages and assigns them numerical value for each amplitude.
• QUANTIZATION: The process of an ADC reading the signal input and assigning each amplitude their binary value.
• DAC: Digital to Analogue Converter. They read the binary signal and convert them to a voltage which is then amplified by an amplifier.
• PCM: Pule Code Modulation. The method that is formally used to represent sound digitally. PCM is broken down into sample rate and bit depth.
• Sample Rate: The # of samples taken per second. CDs are rendered at 44.1 kHz sample rate.
• Bit Depth: The number of bits used to define the amplitude of each sample that determines the resolution with which we can measure the signal. EX: Think pixels in a picture. That's how we attain more clarity within a digital signal.
• Binary Number System: The system we use to communicate with computers. They're registered under 1s and 0s.
•  Nybble: 4-bit string; has 16 possible values; 2^4(2 to the 4th power) Byte: 8-bit string; has 256 possible values; 2^8(2 to the 8th power)
• PCM based digital audio uses these values as representation for the total resolution an amplitude of a wave can be measured.
• Bit: The smallest unit of information which all classes of Bytes consist of.
• DSD: Direct Stream Digital; A 1-bit digital audio format that uses a sample rate of 2.8224 mHz. It averages out the difference between two samples to predict the value of the next sample.
• SACD: Super Audio Compact Discs; uses DSD along with technology from Sigma Delta Modulation. (abbreviation is DeltaSigma)
• Comparator: Analyzes the difference between the actual voltage and the expected voltage and assigns a 1-bit value to express if the real voltage is greater or less than expected value.
• MOST DAWS DON'T USE DSD, SACD, OR OTHER DELTASIGMAS
• Some audio converters use DeltaSigma modulation followed by a decimator that allows the information to be converted to PCM.
• Nyquist Theorem: The sample rate or fqz of samples must be at least two times the highest frequency being recorded in order to accurately capture and encode all fqz in the signal. AKA The computer can only accurately represent fqz up to half the sampling rate.
• Nyquist FQZ: 1/2 of the selected sample rate.
• Aliasing: The result of fqz being misidentified and their values being applied to the incorrect frequencies. Unless an exact spectrum of a sound is known, it is hard to distinguish between aliasing frequencies and properly digitized frequencies
• Anything above the Nyquist frequency is "folded back" and processed unintentionally. EX: 16k samples = max. 8 kHz frequencies. 9 kHz would be processed as 7 kHz.
• 44.1 kHz is the standard sample rate because it allows us to capture the full range of human hearing, 20 Hz to 20 kHz, and still provide a guard band.
• The Guardband for CDs is between 20 kHz and 22.05 kHz and a low pass filter is applied to anything above 22.05khZ
• LP filter is needed for the output signal of a DAC as well because of the discrete samples having sharp edges.
• Quantization Error: The error that occurs due to lack of bit depth. EX: An 8-bit sample will be off by some random amount from 0 - 1/512th.
• You can reduce quantization error by applying more bit depth for the computer to more accurately analyze the amplitude
• Quantization Noise: Due to the imprecise nature of quantization, the errors it produces in the digital message is emitted as white noise because it is random. (rounded up or down)
• SQNR: Signal-to-quantization-noise-ratio; the ratio of the max. signal amplitude to the max. quantization error.
• The SQNR is equal to 2^n; where n is the number of bits used to quantize each sample. Expressed in dB
• Each bit of precision used in quantization adds 6 dB to the SQNR. EX: 8-bit = 48 dB SQNR. 16-bit (CD QUALITY) = 96 dB SQNR
• Clipping: When the input signal exceeds the maximum amplitude it can be expressed numerically. It will then be "Clipped off"
• Clipping adds distortion with increase in amplitude due to the signal surpassing the maximum amplitude of the transducer.
• The max amplitude of the ADC must not surpass the max. of the DAC
• DBFS: dB relative to full scale. 0 dbfs is assigned to the highest possible value represented by its bit depth. a signal half the highest value would be -6 dbfs.
• Calibration is used to match ADCs with DACs. A converter calibrated to -18dbfs indicates an analog signal of 0dbu = -18dbfs. 0 dbfs =+18 dbu
• Lower calibration levels create more headroom. Higher calibration can capture a wider dynamic range
• DAT: Similar to VHS; support SR between 32-48 kHz and a resolution of 12 or 16 bits.
• DTRS: HI-8 Cassette as the medium to which eight tracks could be simultaneously recorded at 44.1k or 48k in 16-bit. Later models recorded at 24-bit.
• Hard Track Recorders: 24 tracks of 24-bit digital audio at 44.1k-48k.
• The Digital Audio Stationary Head: DASH; reel tape machine which recorded up to 24-48 tracks of digital audio. It interleaves the digital code over several tracks. You could edit easily by splicing the tape.
• Random Access Digital Audio Recorder: RADAR; One of the first tape-less digital audio recorders. 24 tracks of audio recording at 48k/16 bit. RADAR 6 incorporates higher sample rates (192k)
• Pro Tools: first digital recording studio; Industry Standard
• Recording digital audio to a hard drive that connects via USB 3.0, Firewire, Thunder Bolt, or SATA is standard procedure.