BAV Notes01

Reverberation

Reverberation, in psychoacoustics and acoustics, is a persistence of sound after the sound is produced. A reverberation, or reverb, is created when a sound or signal is reflected causing numerous reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space – which could include furniture, people, and air.

Reverberation time

Reverberation time is the time required for the sound to “fade away” or decay in a closed space after the source of the sound has stopped.

Characteristics of Microphone

Directivity/Pickup pattern/Polar pattern

Sensitivity

 

Frequency response

Low-pass filter (LPF) and  High-pass filter (HPF)

A low-pass filter (LPF) is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies greater than the cutoff frequency. The exact frequency response of the filter depends on the filter design. The filter is sometimes called a high-cut filter, or treble-cut filter in audio applications. A low-pass filter is the complement of a high-pass filter.

A high-pass filter (HPF) is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency depends on the filter design. A high-pass filter is usually modeled as a linear time-invariant system. It is sometimes called a low-cut filter or bass-cut filter.

Analog Videotape Recording Principles

A real problem when we try to record full blown video using a method similar to what we do with audio. Basic physics states that the highest frequency you can record on magnetic tape is determined by two things.

·        One is how fast the tape travels by the head,

·        and the other is how narrow the head gap is (the space between the two poles on a recording head).

The formula is :                  F_max = V_tape / 2 x W_gap.

·        This means that the maximum frequency you can record is equal to the velocity of the tape as is passes by the head, divided by twice the width of the tape head's gap

·        In audio, the highest frequency that has to be recorded is about 20,000 cycles per second.

·        This means that, at 15 inches per second tape speed, It will need a head gap of .000375 of an inch (20,000 = 15 / 2 x W_gap, or W_gap = 15 / 2 x 20000 = .000375.) 

·                          The video signal has frequencies up to 4.2 million Hz in it. Using the formula,                 we get 4200000 Hz x 2 x .00006 gap width = 504 inches per second.           ·     A half hour recording at this speed would require 75,600 feet of videotape                (a little more than 14 miles).

Growth and Decay of sound in an enclosure

An enclosed space is a room or area bounded on every of its sides. The materials for enclosure may be classified into two:

• Those that allow sound rays to pass through and

• Those that do not allow sound rays to pass through.

When a sound source releases sound energy, the intensity of sound suddenly increased when measured at any particular point. This intensity is caused by the direct sound that reaches the measurement point and is dependent on the distance between the source and the destination.

Indirect sound is caused by reflections from the various surfaces within he enclosure and is not a factor distance between the source and the destination. The total sound level is the sum of the direct sound and will keep increasing till the equilibrium.

When sound sources switched off suddenly the sound intensity will gradually die away and not end suddenly. But the indirect song keeps bouncing around the enclosures various surfaces that absorb some of the energy before it reaches the measurement point a little after the directive sound.

Sound decay can take a long time as it is dependent on the shape of the enclosure and the amount of absorbent material used and how it is positioned in the enclosure. The term used for the gradual decay of sound energy is reverberation .

             The Difference Between a Graphic and a Parametric Equalizer

Equalization in the process of balancing frequency components with an electronic signal and is very popular in sound recording and reproduction. An Equaliser is the equipment that facilitates equalization by strengthening or weakening in the energy of specific frequency band. Graphic and parametric equalizers have much more flexibility in modification of the frequency content of an audio signal. Since equalizers "adjust the amplitude of audio signals at particular frequencies.

Graphic equalizer

In the graphic equalizer, the input signal is sent to a bank of filters. Each filter passes the portion of the signal present in its own frequency range or band. The amplitude passed by each filter is adjusted using a slide control to boost or cut frequency components passed by that filter. The vertical position of each slider thus indicates the gain applied at that frequency band, so that the knobs resemble a graph of the equalizer's response plotted versus frequency.

The number of frequency channels (and therefore each one's bandwidth) affects the cost of production and may be matched to the requirements of the intended application.

Parametric equalizer

Parametric EQ.

Parametric equalizers are multi-band variable equalizers which allow users to control the three primary parameters: amplitude, center frequency and bandwidth.

·        The amplitude of each band can be controlled,

·        and the center frequency can be shifted,

·        and bandwidth (which is inversely related to "Q") can be widened or narrowed.

Parametric equalizers are capable of making much more precise adjustments to sound than other equalizers, and are commonly used in sound recording and live sound reinforcement. Parametric equalizers are also sold as standalone outboard gear units.

Tape bias

Tape bias is the term for two techniques, AC bias and DC bias, that improve the fidelity of analogue tape recorders. When recording, magnetic tape has a nonlinear response as determined by its coercivity. Without bias, this response results in poor performance, especially at low signal levels. A recording signal which generates a magnetic field strength less than tape's coercivity is unable to magnetise the tape and produces little playback signal. Bias increases the signal quality of most audio recordings significantly by pushing the signal into more linear zones of the tape's magnetic transfer function.

DC bias

The earliest magnetic recording systems simply applied the unadulterated (baseband) input signal to a recording head, resulting in recordings with poor low-frequency response and high distortion. Within short order, the addition of a suitable direct current to the signal, a DC bias. The principal disadvantage of DC bias was that it left the tape with a net magnetization, which generated significant noise on replay because of the grain of the tape particles. Some early DC-bias systems used a permanent magnet that was placed near the record head. It had to be swung out of the way for replay. DC bias was replaced by AC bias but was later re-adopted by some very low-cost cassette recorders

AC bias

AC bias is the addition of an inaudible high-frequency signal (generally from 40 to 150 kHz) to the audio signal. Most contemporary tape recorders use AC bias.

    

 Mixers

The mixing console or "mixer" is a central component of most sound systems. In fact, the mixer used will have a large influence on the operability and efficiency of the entire system.

Digital Mixers

In digital mixers audio signals are converted to and processed in digital form. This type is more expandable, meaning you can connect a greater number of external devices.

Advantages of a digital mixer are

(1) Settings can be pre-programmed and recalled when necessary

(2) Mixing and processing features that only digital can provide

 (3) Expansion and external devices

(4) Noise-resistant digital transmission

(5) Multiple functions in small spaces

Analog Mixers

Analog mixers work in the opposite way of digital mixers — this type of mixer uses analog sound transmission instead of digital sound transmission.

Analog mixers are much easier to operate because they have only one function per control.  The more simple design is better suited for those learning how to use mixers. Analog mixers are also less expensive than digital mixers.

MPEG

MPEG stands for Motion Pictures Expert Group. It is ISO standard for Video and Audio. MPEG standard also support online Audio and Video. MPEG also refer the family of Digital Video Compression standard and file formats of this group MPEG algorithm show better compression so video data can be eaisly communicated through on line channel and easily decompressed at receiver site.

MPEG follow lossy compression Technique for the compression of Video data. The video data which is not relevant to visualization of human eyes these data is removed from the video that’s why Video Compression follow lossy compression MPEG show high compression rate because it store difference of frames rather than entire frame of video.

MPEG1

·        Standard for lossy video/audio compression Developed by ISO,IEC. Designed to compress VHS-quality raw digital video  and CD audio down to 1.5 Mbit/s Provide coding of Video and its associated Audio with speed 1.5 MBPS for digital storage media. It is an extension of JPEG ,H.261. Only supports progressive pictures.

 

Applications/Advantages

·        Most popular s/w for video playback includes MPEG-1 decoding, in addition to any other supported formats. 

·        The popularity of MP3audio has established a massive installed base of hardware that can play back MPEG-1 Audio (all three layers). 

·        "Virtually all digital audio devices " can play back MPEG-1 Audio.Many millions have been sold to-date.

·        Before MPEG-2 became widespread, many digitalq satellite/cable TV services used MPEG-1 exclusively.

·        The Super Video CD standard, based on VCD, uses MPEG-1 audio exclusively, as well as MPEG-2 video.

·        Most DVD players also support Video CD andq MP3 CD playback, which use MPEG-1

·         The international Digital Video Broadcastingq (DVB) standard primarily uses MPEG-1 Layer II audio, and MPEG-2 video.

Drawback of MPEG 1

·        Only support Progressive scanning

·        Do not support broadcasting

·        Low picture quality

·        Achieve Low compression ratio.

MPEG2

MPEG2 is used for DVDs, ATSC (High Definition Television) broadcasts, Personal Video Recorders (such as TiVo), and many other applications.  MPEG2 is so versatile that while originally it was planned to have an MPEG3 standard for High Definition TV broadcasts, it turned out that MPEG2 scaled in terms of bit rate so that only 1 standard was necessary for both Standard and High Definition video.

 

Advantages

·        MPEG2 gives better picture quality than MPEG1 at full CCIR 601 resolution (720x480) and at comparable bit rate. 

·        MPEG 2 support scaling but MPEG1 not support it.

·        MPEG 2 Support both Progressive and Interlaced Scanning but MPEG1 support only Progressive scanning.

Disadvantage of MPEG2

·        MPEG2 takes more CPU horsepower to decompress, and basically has no advantage over MPEG1 at lower resolutions. 

·        If you're distributing for the web, you're not going to want to be compressing stuff in MPEG2 instead of MPEG1 because the benefits of MPEG2 aren't realized until you get into full TV resolution, which means big file sizes.

MPEG4

MPEG-4 is a new multimedia standard that is designed for use in broadcast, interactive and conversational environments. The way MPEG-4 is built allows MPEG-4 to be used in Television and Web environments, not just the one after the other, but also facilitates integration of content coming from both channels in the same multimedia scene.

Advantages

·        Integration of natural and synthetic content, in the form of objects. Such objects could represent 'recorded' entities (a person, a chair) or synthesised material (a voice, a face, an animated 3D model).

·        Support for 2D and 3D content.

·        Support for several types of interactivity.

·        Coding at very low rates (2 Kbit/s for speech, 5q Kbit/s for video) to very high ones (5 Mbit for transparent quality Video, 64 Kbit/s per channel for CD quality Audio). Support for management and protection ofq intellectual property.

·        MPEG-4 provides DVD quality video, but uses lower bit rate in comparison to MPEG 2 so that it's feasible to transmit digitized video streams in MAN , and also in WAN, where bandwidth is more critical, and hard to guarantee.

·        Support video conferencing over Internet.

·        MPEG-4 preserves compatibility with major existing standards: MPEG-1, MPEG-2, ITU-T H.263, and VRML.

·        Good video quality at reasonable bitrates

·        Excellent cost/performance

·        Near universal support by VMS

·        Low latency