Question

Consider a DASH system for which there are \(N\) video versions (at \(N\) different rates and qualities) and \(N\) audio versions (at \(N\) different rates and versions). Suppose we want to allow the player to choose at any time any of the \(N\) video versions and any of the \(N\) audio versions. a. If we create files so that the audio is mixed in with the video, so server sends only one media stream at given time, how many files will the server need to store (each a different URL)? b. If the server instead sends the audio and video streams separately and has the client synchronize the streams, how many files will the server need to store?

Short answer

a. The server needs to store \( N^2 \) files. b. The server needs to store \( 2N \) files.

Step-by-step solution

Understanding the Problem

In a DASH system, we have different versions for video and audio streams. We need to determine how many distinct files are needed in two different scenarios: first, where audio and video are combined into one file, and second, where they are stored separately.

Scenario A: Combined Media Stream

In this scenario, each file contains a combination of one video version with one audio version. Since there are \( N \) video versions and \( N \) audio versions, and the player can choose any combination, we calculate the total number of combinations as \( N \times N = N^2 \). Therefore, the server needs to store \( N^2 \) files.

Scenario B: Separate Media Streams

Here, the audio and video are stored separately, and each stream is synchronized by the client. The server needs to store each version of the video and each version of the audio separately. Therefore, the server will store \( N \) video files and \( N \) audio files, resulting in a total storage of \( 2N \) files.

Key concepts

Media Streams

In the context of a Dynamic Adaptive Streaming over HTTP (DASH) system, media streams refer to the different types of content—like video and audio—that are sent over the internet for playback on demand. Streaming is an efficient way to deliver media as it allows a user to start watching or listening almost immediately without waiting for the entire file to download. This is particularly useful for large files, such as videos in multiple resolutions or audio tracks.

Media streams can be delivered in one of two ways within a DASH system:

• Combined Streams: Video and audio are integrated into one single stream.
• Separate Streams: Video and audio are separated into individual streams that are synchronized during playback.

Each approach has implications for the amount of storage needed and the complexity of syncing media during playback.

Video Versions

When it comes to DASH systems, video versions are available in multiple quality levels. These levels accommodate different internet speeds and devices, ensuring a smooth playback experience no matter the conditions. Each version is optimized for a specific bitrate and resolution.

The concept of video versions is crucial because:

• Users with high-speed internet can enjoy ultra-high-definition versions.
• Those with slower connections can select lower resolution versions, reducing buffering time.
• Such adaptability maximizes viewing quality across a variety of devices, from large screens to mobile phones.

In our exercise, having \( N \) video versions means that there are \( N \) distinct files or representations for the video content.

Audio Versions

Audio versions in a DASH system are similar to video versions. They refer to the various quality or language options for the audio component of the stream. This could range from high-quality stereo sound to more compressed audio options that consume less bandwidth.

The reasons for having multiple audio versions include:

• Accommodating different user preferences for language or audio quality.
• Providing optimal audio experiences on varying devices, such as headphones or built-in speakers.
• Ensuring that even users with bandwidth constraints can access consistent audio alongside different video streams.

In this exercise, there are \( N \) distinct audio versions available, which contributes to the flexibility seen in the separate streaming approach.

File Storage

File storage in DASH systems is a key consideration when managing media streams. Efficient storage ensures that all combinations of video and audio streams are readily available to match user selections.

In the exercise, two main storage strategies are discussed:

• Combined File Storage: Each file contains a mix of one video and one audio version. This configuration, though simpler for streaming, requires \( N^2 \) files in total. Every combination of audio and video must be pre-combined into an individual file.
• Separate File Storage: Video and audio streams are stored independently. The server needs to manage \( 2N \) files, one for each version of video and audio. This offers more flexibility, as any combination of video and audio can be synchronized by the client during playback.

Choosing the right storage method balances between efficiency (in terms of storage space) and flexibility, impacting buffer times and server load management.