Interview Questions-differences between MPEG4 and H.264

I. Audio compression encoding standard.

1. mpeg1 Standard

Mpeg1 audio compression algorithm features:

1) the input signal of the encoder is a linear PCM signal. The sampling frequency is 32 kHz, 44.1khz or 48 kHz, and the output bit rate is 32-384kbit/s.

2) the compressed bit stream supports both single and dual channels.

3) The mpeg1 audio compression standard provides three independent compression layers: Layer 1 (layer1), Layer 2 (Layer2), and Layer 3 (layer3)

4) MP3 is a MPEG1-Layer3 standard, with a compression ratio of to CD

 

2. MPEG2 standard

The MPEG-2 defines two audio compression encoding algorithms, MPEG-2 BC and MPEG-ACC.

1) MPEG-2 BC: is compatible with the MPEG-1 audio compression coding algorithm, is a multi-channel surround sound compression coding standard, support 5.1 and 7.1 sound ring sound. Due to the need for forward compatibility, the algorithm has some defects and is blocked in the application.

2) MPEG-ACC: not compatible with MPEG-1 audio compression encoding algorithm. AAC supports 48 main channels, 16 low-frequency enhancement channels, 16 dubbing channels, and 16 data streams ., The compression ratio is 11:1.

 

3. MPEG-4 standards

The goal of MPEG-4 standards is to provide future interactive multimedia applications with high flexibility and scalability.

 

4. AC-3 standards

1) developed by the Dolby laboratory in the United States, there are six completely independent sound channels

2) AC-3 encoder inputs PCM sound data and outputs compressed data streams.

 

Ii. Video compression coding standards.

1, MPEG-2 is not a simple upgrade of MPEG-1, it in the system and transmission has made more detailed provisions and further improve. It consists of nine parts:

1) system. This section describes how to synchronize multiple basic video and audio streams (ES) and additional data synthesis transfer streams (TS) and program streams (PS) in real time.

2) video. Describes the encoding and decoding of video data.

3) audio. MPEG-2 BC describes the encoding and decoding of audio data, backward compatible ..

4) consistency test.

5) software simulation.

6) digital storage media-command and control extension protocol.

7) MPEG-2 Advanced Audio Encoding (AAC), is a multi-channel audio encoding standard.

 

2. the MPEG standard divides images into three types: I frame, P frame, and B frame.

1) I frame: only the spatial correlation in the image is used, but not the temporal correlation. Therefore, the I frame image compression ratio is relatively low. I frame can be used as a reference frame for B frame and P frame prediction.

(2) P frame: Forward prediction is performed based on the I frame image or P frame image closest to the front. Meanwhile, the spatial and temporal correlations are used, and the compression ratio of P frames is relatively high.

3) B frame: it is a bidirectional prediction interpolation frame. Frame B cannot be used as a reference frame for Motion Compensation prediction of other frames.

 

3. Where is H.264? Let us first MPEG-2, MPEG-4 (MPEG-4 part 2), H. 264 (MPEG-4 AVC, MPEG-4 Part 10) Three resolution performance and the use of bandwidth, regardless of the MPEG-2, MPEG-4, H. 264, all three can achieve 1920 x I (non-staggered) high definition (HD), 24 FPS (24 images updated per second) image quality, but the MPEG-2 on the transmission bandwidth needs 12 ~ 20 Mbps, the relative H.264 value is 7 ~ 8 Mbps, while the MPEG-4 is between the two, more directly, if the MPEG-2 bandwidth is used as the benchmark of 100%, MPEG-4 to achieve the same effect only needs 60% bandwidth, H. 264 is as low as 40%, about 1/2 of the original MPEG-2 ~ 1/3.

 

3. System Transfer Layer of MPEG-2

1. Es stream: After video and audio signals are encoded, their basic streams are generated.

3. PES stream: The elasticsearch stream for video and audio is respectively packaged in a certain format. The length can be within a certain range.

2. TS Stream/PS stream: Pack and reuse video, audio PES, and auxiliary data in different formats to generate ts streams and PS streams respectively. (TS is for broadcast applications and PS is for local applications)

 

Iv. image compression coding standards.

1. JPEG: it adopts a special lossy compression algorithm to delete the color of images that are not easy to be noticed by the human eye, so as to achieve a large compression ratio (up to or even 40: 1 ), JPEG is the most widely used format on the Internet because the size of the file is small and the download speed is fast. The color quality is up to 24 bits (16700000 colors ).

2. GIF: The biggest feature is not only a static image, but also an animation. It also supports transparent background images. It is suitable for multiple operating systems and has a small size, many small animations on the Internet are in GIF format. However, the color gamut is not very wide, and only 256 colors (8 bit) are supported.

3. Bitmap: developed by Microsoft, it is a basic image format for Windows and OS/2 and can be opened by all Windows programs. In this format, the image information is saved pixel by pixel. Therefore, the BMP format is suitable for saving the original images with all the details, but the file in this format is huge.

4. PNG: Like JPEG, PNG supports 24-bit colors. Like the GIF color palette, PNG can also define the color as transparent, so that the logo can avoid interfering with the background. The biggest difference with JPEG is that it will not cause quality loss when compressing the image, but the size of the corresponding file will be larger than GIF and JPEG. PNG has an additional feature that is ahead of two competitors. This is the ability to store Attachment Information in images, such as information about the author.

 

5. Differences between a vector image and a bitmap:

1) vector graphs use straight lines and curves to describe graphs.

2) bitmap is a set of pixels.

 

V. Digital TV standards:

DVB-T, DVB-S, DVB-C

DMB-T China)