MODULE -17
Audio/Video interface
to a microcontroller/embedded system:
Need of an audio and video interface: If you look
at the way human naturally communicate; it's by voice and
the best way to explain any subject is to show the real
picture i.e. video. The present electronics communications
can be more natural if they handle audio and video data
in all aspects (Both as data and commands). In fact today's
digital device much more capable in accepting audio and
video input and also outputting audio and video to speaker
and LCD display or any display device. However streaming
audio and video in/out of a processor is not that easy task.
This article helps you in understanding designing audio/video
interface to an embedded system.
The audio and video signal (analog and digital):
Audio: Audio in the form of sound energy is converted
into electrical signal by using different types of microphones
(the most popular now are electret/condeser microphone and
MEMS microphone). The video in the form of light is converted
into electrical signal using image sensors (the key component
inside digital camera).
Analog vs Digital audio (comparison): The analog
and RF electronics engineers have found it challenging to
provide high quality audio over radio. However best the
audio is converter to electrical signal, different types
noise were added to the audio signal while it is getting
amplified, transmitted, stored and retrieved, so that over
the time the audio looses its originality. This is the problem
with analog signal. Where as a digital signal, either it
is signal or no-signal (1 or 0). Even the noise modulates
or ride over the digital signal it does't impact the signal
o/p as long as it is detectable as '0' or '1' (as long as
noise stays below threshold). So digitizing audio into logic
'1' and '0' helped to get rid of all major noise except
one noise called quantum noise, which caused least effect
to the fidelity of the audio. The audio once digitized it
is easy to store in digital format and easy to copy multiple
copies without adding any noise to it.
Digital audio is nothing but binary coded audio. See the
picture below explaining one method of converting analog
audio to digital audio.
Video: In case of video the picture in the form
of light energy is converted to electrical energy using
CCD or CMOS image sensors. They produce electrical analog
signal in proportional to the red green yellow colors. Handling
video data is much complex compared to audio due to sensor
complexity, multiple signals and also the signal bandwidth,
just like the difference between our ears and eyes. The
video signal is not just plain signal it also carries sync
signal for the display to sync the picture frame. The analog
video spec is based three popular specs called PAL, NTSC
and SECAM.
Audio specifications:
Complete audible frequency range: 20 to 20,000 Hz
Voice range: 300 HZ to 3400 Hz
Video Specifications:
Frame rate: 25 frames to 120 frames per second
Bandwidth: 4- 5 MHz for analog video (PAL/ NTSC and SECAM)
For High Definition (HD) video: in the range of 200 MHZ
The video signal falls in the RF range and complexities
are higher. Visit the link below to know common analog video
signal characteristics:
http://www.onsemi.com/pub_link/Collateral/AND8261-D.PDF
Use the software in the link below to calculate bandwidth
for different video signals:
http://www.csgnetwork.com/videosignalcalc.html
So the raw video signal and audio signals coming out of
the sensors are plain analog signals except for frequency
the bandwidth difference. Digital audio and video is again
much similar except the procedures used for encoding, compressing
and decoding.
Encoding, compressing and decoding: Converting of plain
analog audio/video signal into digital signal is called
encoding. There are various encoding standards available.
Compressing is done to reduce the size of the encoded digital
signal so that the repetitive and blank signals are stored
with a reduced digital code. Decoding is a procedure to
convert the compressed digital signal into analog signal.
The first stage of digitizing is done through a analog
to digital converter IC/chip (ADC) and the final stage of
retrieving the analog signal is done using Digital and Analog
Converter IC/chip. The rest in between i.e. further coding,
compressing and decoding is achieved through processor using
software programs called codecs. The latest and most used
encoding standards for digital video is H.264 and MPEG.
Understanding H.264 and MPEG takes more time, we will cover
that in our next module. Meanwhile you can refer to the
wikipedia.org (search for H.264 and MPEG) which provides
good introduction to H.264 and MPEG.
Interfacing audio/video to a DSP/processor or advanced
microcontroller (MCU) with DSP capabilities:
Now let's get into how to interface audio and digital audio
to microcontroller/processor. Mainly for the microcontroller
to handle audio it needs DSP engine or an external audio
processor chip. Below is the block diagram of audio interface
to a microcontroller/embedded system.
In the above diagram the microphone is connected to pre-
amplifier and then to a input of a Analog to Digital Converter
(ADC chip/IC). The digital output of ADC is fed to data
bus of processor and the ADC is controlled through processor.
The processor can be exclusive DSP chip or a high performance
general purpose processor or a DSP capable Microcontroller.
ADC can be an external device or it can be on-chip ADC in
the microcontroller/processor chip.
The digitized audio from the ADC after once it enters the
processor, it can be encoded further for any audio format
such as wav, mp3, wma using a software program called codec.
The encoded data can be stored on a SD card or any such
flash memory based storage connected through SPI interface.
The retrieval is just the opposite of storing. The data
from SD card is decoded by the processor and fed to a Digital
to Analog converter (DAC chip) and to a speaker (again see
above diagram). In stereo-audio the number of analog audio
channels are two.
In case of video interface the microphone is replaced by
a image sensor and the three light outputs RGB are connected
to 3 different ADC chips before feeding to the data bus
of processor .The rest of process is same as Audio where
H.264 and MPEG codecs are used to store the data in H.264
and MPEG format. The video retrieval procedure is same as
audio retrieval except for the increase in complexity, number
of channels, and the higher bandwidth. Some of latest microphones
have built-in analog and digital circuits to deliver the
diigital audio directly.
The audio and video can together be stored and retrieved
using modern codecs.
Here below provided are the links to nice articles on
this subject for further reading:
By opening the application note by visiting this link below
you can design a voice recorder using Atmel Micrcontroller
AVR AT90S8535 and dataflash card. This has complete voice
recorder circuit diagram. The name of application note is
"AVR335: Digital Sound Recorder with AVR and DataFlash".
The theory of operation is well explained.
The link is http://www.atmel.com/dyn/resources/prod_documents/doc1456.pdf
If you wish to learn how the video signal from the image
sensor is handled by ADC, there is a exclusive video ADC
from NXP Semiconductor called TDA8754. The datasheet available
in the link below can help you to understand how the RGB
channels are connected to ADC. The title of the data sheet
is "TDA8754 Triple 8-bit video ADC up to 270 Msample/s"
http://www.nxp.com/documents/data_sheet/TDA8754.pdf
You can learn about specifications and characteristics
of NTSC, PAL and SECAM video signals in this application
note titled "Introduction to Analog Video" by
ON Semiconductor at the link http://www.onsemi.com/pub/Collateral/AND8261-D.PDF
The video is playing increasing important role in making
a digital product successful. To know some of market facts
and trends on importance of video in electronics systems,
read this ppt from imagination technologies at the url below.
http://www.mips.com/seminar2011/PM-1-Imagination_MIPS_Computex_2011.pdf
ARM and MIPS processors are capable of handling audio and
video effectively and so is gaining popularity in use for
various convergence application. A paper titled "Developing
Video Phones with ARM Processor-based Solutions" at
the link below explains how ARM processor used for video
phone application.
http://www.iqmagazineonline.com/magazine/pdf/v_4_4_pdf/v_4_4_pg-16-19.pdf
