Video Phone Introduction

This article briefly introduces the composition and structure of the videophone subsystem and briefly introduces the Android-based videophone. Each manufacturer's Videophone Implementation Scheme is slightly different from each other, and cannot be totally different.

Contents [Hide] 1 1 Android Videophone 2 2 Introduction to videophone protocol stack 2.1 2.2 H.245 Protocol 3 3 Videophone creation process 3.1 3.1 Create a CS domain videophone call 3.2 3.2 3g-324m negotiation Establishment 3.3 3.3 Videophone Data Transmission 4 Android Videophone Implementation

[Edit] 1
Android Videophone

As shown in, Android videophone consists of the above modules, which are based on wireless communication services and have extremely high requirements for multimedia processing. Control camera. The microphone captures audio and video data of the current end in real time and sends it to the network through 3g-324 protocol stack encoding. At the same time, push the audio and video data from the peer end to the LCD and speaker. Because the proprietary videophone protocol is used, it is essentially different from http data service transmission.

This article focuses on the following modules:

Vtapp:It can be added and modified on the Android phone app. The complexity of this method is reduced.

Vtengine:It is also considered as the Adaptation Layer of VT. This module can be stored in framework Java or library. The specific layer involves audio and video data shipping and other issues, so it is assumed that this is one reason why MTK's visual phone vtengine is placed on the Lib layer. Encapsulate 3g-324m and provide the upstream interface. Call the camera and audio interfaces to collect local audio and video data. It is also responsible for releasing the remotely decoded audio and video data.

3G-324 M:The following sections describe in detail, which is simply an essential protocol for implementing videophone. In Android external.

VT Port:Android phones generally use dual-chip design. There are two data communication methods between the modem side and the AP side, one is the port Port port. The other is dual-port Memory RAM. If the port is used, at least two ports are used to transmit at commands in the CS domain and data related to data transmission in the PS domain. The visual phone needs to add a VT port. This function is used to transmit local and peer visual phone data on the AP and modem sides.

[Edit] 2
Introduction to videophone protocol stack

After having an overall understanding of videophone, let's look at what is a strange videophone protocol stack? What components does it have and how can we work together?

Based on ITU h.324m and other international standards, 3GPP has developed a 3g-324m Framework Protocol which can implement real-time multimedia services on the wireless circuit switching network. Enables video, audio, and control signals to be transmitted on the same 64 K physical link. 3g-324m is a standard system that includes the following sub-protocol standards: speech coding standards (audio pp-amr, g.723.1), video coding standards (H.263, MPEG-4, H.261) data encoding standards (V.14, lapm (Link Access Procedure for modems), multiplexing and separation standards (h.223), and call control standards (H.245 ).

　　

H.223: multiplexing and separation of voice, video, user data, and control data (multiplexing, de-multiplexing ), the application standards for multiplexing of multiple audio and video signals in a single mobile communication channel have been developed;

H.245 sets the message control exchange standards at various stages;

Audio Codec, arm is the mandatory standard, and g.723.1 is the optional old encoding standard;

Video Codec, H.263 as mandatory benchmark protocol, MPEG-4 is the recommendation standard;

[Edit] 2.2
H.245 Protocol

In 3g-324m, H.245 is used as the terminal control protocol. The minimum supported H.245 version is version 3. We recommend that you implement an updated version.

H.245 provides the following functions:

Support for NSP and ccsrl;

Determine the master-slave mode. At the beginning of the session, determine which terminal is used as the master control terminal. When a conflict occurs, the master terminal determines the communication status;

Terminal capability exchange, such as optional mode of multiplexing, codec, data sharing mode and parameters;

Initialize and update reusable tables, and add, delete, and reuse table items;

Mode request is used to request the operation mode from the receiver to the sender;

Transmission Delay measurement to provide precise quality feature measurement;

Loopback testing );

Miscellaneous call control commands and instructions.

[Edit] 3
Videophone creation process

[Edit] 3.1
Create a CS domain videophone call

VT's CS domain call creates a voice call similar to that on Android. The native Android Code does not provide a videophone interface. Therefore, you need to add an API to framework telephony, add the videophone interface and AT command to the RIL Hal layer. After a CS domain call is established, a 64 kbit/s transparent data transmission link is also established. The 3g-324m bit stream is transmitted through this link.

[Edit] 3.2
3g-324m negotiation Establishment

1. Mobile phone level detection: h.223 multiplexing layer: each mobile phone sends a flag sequence to indicate the highest supported mobile multiplexing level. The highest value is used as the public level; mobile phone levels 1, 2, and 3 correspond to h.223 accessories A, B, and C respectively;

2. Transmission of MUX-PDUS: Transmission of h.223 multiplexing protocol data unit frame;

3. H.245 simple response protocol (SRP );

4. Terminal exchange capabilities, including audio and video capabilities, supported mobile extensions, and supported reuse options;

5. Determine the master-slave mode. The H.245 message "master slave determination (MSD) request" determines the master-slave mode of the terminal;

6. Open the Logical Channel, carry voice and video media, and define their reuse tables to the remote demultiplexing end;

7. Establish a call for media exchange.

[Edit] 3.3
Videophone Data Transmission

After the establishment of 3g-324m negotiation, media data exchange starts. As shown in, the raw data collected by camera is imported into 3G-324 M from the audio data collected by the microphone. It is encoded and reused. Transfer to vtport. The modem then sends the data to the network through an air interface.

After receiving the data from the modem, the peer data is transmitted to 3g-324 through the vtport. After the data is deduplicated and decoded, the original data is pushed to the surfaceflinger display, and audio is played by the speaker.

In addition to audio and video data, user data transfers images or text during the videophone process.

Control Data. H.245 not only plays a role when setting up a videophone, but also sends traffic control, General commands, and instructions during the videophone process.

[Edit] 4
Android Videophone Implementation

Android videophone is not a simple Google market application. To implement CKT's own videophone, pay attention to the following aspects:

1. obtain support from the chip solution vendor and negotiate with the chip solution vendor on how the modem side communicates with the AP side to transmit the videophone reused data.

2. Know the AT commands related to the visual phone on the modem side.

3. Select the appropriate 3g-324m videophone protocol stack Lib.