Audio & Midi Protocols 2 Flashcards
Header
At the beginning of each frame is a header that identifies the packet. The heads act as synchronizers, setting up points throughout the mp3 file at which playback can begin in the middle of a song. The headers permit fast forwarding and reverse.
Tag
contains information about the song, including name, artist, album, length and track number. The mp3 player uses this information for its display while the song is playing.
MP3 Data
the rest of the file is audio data, in the form of bits, that has been compressed during the creation of the file
Ogg
Hardware manufactures can support Ogg Vorbis in their designs without having to pay license fees. Users can modify the format’s code for their own needs. It also supports up to 255 distinct channels, instead of just 2.
Typical Bitrates
8 kbit/s Telephone quality (using speech codecs).
32 kbit/s AM Quality.
96 kbit/s FM Quality.
128 kbit/s -160 kbit/s Was the standard bitrate quality.
192 kbit/s DAB Quality.
224 kbit/s - 320 kbit/s Near CD Quality.
Constant Bit Rate
means that each sample of the encoded file must use the same amount of bits.
Average Bit Rate
average amount of data transferred per unit of time, usually measured per second
Variable Bit Rate
This gives the encoder the freedom to save bits on simple sections of audio that don’t need as many bits and vice versa.
Analog Communication
is a type of communication where an analog signal is transferred between two devices through a medium. It can be transmitted through either wired or wireless communication.
Single Core / Shielded Cable
The single core is used for the +ve, or ‘hot’, and the shield is used for the -ve, or ‘cold’. This type of cable is used for unbalanced audio signals.
One Pair / Shielded Cable
This cale has one core as the +ve, and the other core is -ve. The shield is earthed. This type of cable is used for balanced audio signals.
Balanced Audio
works on the principle that two identical signals which are
inverted 180°out of phase will cancel each other out.
Unbalanced Audio Cables
uses two lines to transmit the audio signal -a hot line which carries the signal and an earth line. This is common in short cables (where noise is less of a problem) and less professional applications.
Balanced Audio Cables
Consist of a hot line, cold line and earth. The audio signal is transmitted on both the hot and cold lines, but the voltage in the cold line is inverted therefore the two signals are 180 degrees out of phase with each other.
Digital Communication
Only uses a wired medium for the transfer of data. The digital signal can have multiple discrete values but when they are represented using only two possible values of amplitude they are called a binary signal. The binary signal or data exists in the form of bits whose amplitude is either only “0” or “1”
Digital Interconnects
The binary data exists in the form of bytes where each byte is made of 8 individual bits. Therefore, the digital signal can be sent bit by bit in a sequence or we can send the whole byte at a time.
Synchronous Communication
occurs between two devices when a clock signal is sent along with the data (either on separate wires, or modulated with the data).
Asynchronous Communication
occurs when the sending and receiving devices are not locked directly, but have the same frequency - well suited for applications where messages are generated at irregular intervals.
Isochronous Communication
is where a network has a master clock signal to which all devices are synchronized.
S/PDIF (Sony/Philips Digital Interface)
In case of S/PDIF, the send rate is the sample rate as well. As this rate is generated by the clock of the sender, any fluctuation in the send rate manifest itself as input jitter at the DAC. S/PDIF is therefore a hybrid signal, the data is digital but the timing is analog.
HDMI
the audio clock is derived from the video clock. For high-definition TV, the video clock runs at 74.25 MHz. The audio clock runs at multiples of 48 kHz.
MIDI Time Code (MTC)
is time-based only relating to hours, minutes, seconds and frames, not measures or tempo.
MIDI Clock
is a measure driven pulse (beat) clock. Carries Play, Stop, Forward, and Backward commands as well as tempo data. Ability to slew to tempo changes.
OPEN SOUND CONTROL (OSC)
Typically transmitted over Ethernet. OSC has a binary data section, but a human readable, URL-style address that looks like “/foo/bar”. OSC allows one to transmit multiple data types commonly used on modern computers including 32-bit integers, floating point numbers, strings and more. OSC includes a high-precision timestamp with picosecond-resolution that allows OSC messages to be scheduled, recorded and reproduced with minimal jitter.
