Good Practice Guide for Developers of Cultural Heritage Web Services
Good Practice Guide for Developers of Cultural Heritage Web ServicesThis section was originally published as QA Focus Briefing papers.
The digitisation of digital audio can be a complex process. This document contains quality assurance techniques for producing effective audio content, taking into consideration the impact of sample rate, bit-rate and file format.
Sample rate defines the number of samples that are recorded per second. It is measured in Hertz (cycles per second) or Kilohertz (thousand cycles per second). The following table describes four common benchmarks for audio quality. These offer gradually improving quality, at the expense of file size.
| Samples per second | Description |
|---|---|
| 8kHz | Telephone quality |
| 11kHz | At 8 bits, mono produces passable voice at a reasonable size. |
| 22kHz | 22k, half of the CD sampling rate. At 8 bits, mono, good for a mix of speech and music. |
| 44.1kHz | Standard audio CD sampling rate. A standard for 16-bit linear signed mono and stereo file formats. |
The audio quality will improve as the number of samples per second increases. A higher sample rate enables a more accurate reconstruction of a complex sound wave to be created from the digital audio file. To record high quality audio a sample rate of 44.1kHz should be used.
Bit-rate indicates the amount of audio data being transferred at a given time. The bit-rate can be recorded in two ways - variable or constant. A variable bit-rate creates smaller files by removing inaudible sound. It is therefore suited to Internet distribution in which bandwidth is a consideration. A constant bit-rate, in comparison, records audio data at a set rate irrespective of the content. This produces a replica of an analogue recording, even reproducing potentially unnecessary sounds. As a result, file size is significantly larger than those encoded with variable bit-rates.
Table 2 indicates how a constant bit-rate affects the quality and file size of an audio file.
| Bit rate | Quality | MB/min |
|---|---|---|
| 1411 | CD quality | 10.584 |
| 192 | Good CD quality | 1.440 |
| 128 | Near CD quality | 0.960 |
| 112 | Near CD quality | 0.840 |
| 64 | FM quality | 0.480 |
| 32 | AM quality | 0.240 |
| 16 | Short-wave quality | 0.120 |
Digital video can have a dramatic impact upon the user. It can reflect information that is difficult to describe in words alone, and can be used within an interactive learning process. This document contains guidelines to best practice when manipulating video. When considering the recording of digital video, the digitiser should be aware of the influence of file format, bit-depth, bit-rate and frame size upon the quality of the resulting video.
Digital video consists of a series of images played in rapid succession to create the illusion of movement. It is commonly accompanied by an audio track. Unlike graphics and sound that are relatively small in size, video data can be hundreds of megabytes, or even gigabytes, in size.
The visual and audio information are individually stored within a digital 'wrapper' an umbrella structure consisting of the video and audio data, as well as information to playback and resynchronise the data.
Digital video remains a complex area that combines the problems of audio and graphic data. When choosing to encode video the designer must consider several issues:
The distribution method will have a significant influence upon the file format, encoding type and compression used in the project.
Removeable media - Video distributed on CD-ROM or DVD are suited to progressive encoding methods that do not conduct extensive error checking. Although file size is not as critical in comparison to Internet streaming, it continues to have some influence.
The compression type is dependent upon the need of the user and the type of removeable media:
| NAME | PURPOSE OF MEDIA | Compression | ||
| Streaming | Progressive | Media | ||
| Advanced Streaming Format (ASF) | Y | Temporal | ||
| Audio Video Interleave (AVI) | Y | Temporal | ||
| MPEG-1 | Y | VideoCD | Temporal | |
| MPEG-2 | Y | DVD | Temporal | |
| QuickTime (QT) | Y | Y | Temporal | |
| QuickTime Pro | Y | Y | Temporal | |
| RealMedia (RM) | Y | Y | Temporal | |
| Windows Media Video (WMV) | Y | Y | Temporal | |
| DivX | Y | Amateur CD distribution | Temporal | |
| MJPEG | Y | Spatial | ||
Table 1: A comparison list of the different file formats, highlighting their intended purpose and compression method.
The provision of video data for an Internet-based audience places specific restrictions upon the content. Quality of the video output is dependent upon three factors:
| Screen Size | Pixels per frame | Bit depth (bits) | Frames per second | Bandwidth required (megabits) |
| 640 x 480 | 307,200 | 24 | 30 | 221.184 |
| 320 x 240 | 76,800 | 16 | 25 | 30.72 |
| 320 x 240 | 76,800 | 8 | 15 | 9.216 |
| 160 x 120 | 19,200 | 8 | 10 | 1.536 |
| 160 x 120 | 19,200 | 8 | 5 | 0.768 |
Table 2: Indication of the influence of screen size, bit-depth and frames per second has upon required bandwidth
When creating video, the designer must balance the video quality with the facilities available to the end user. As an example, an 8-bit screen of 160 x 120 pixels, and 10-15 frames per second is used for the majority of content found on the Internet.
Video presents numerous problems for the designer caused by the complexity of formats and structure. Problems may include:
Temporal Compression - Reduces the amount of data stored over a sequence of frames. Rather than describing every pixel in each frame, temporal compression stores a key frame, followed by descriptive information on changes.
Spatial Compression - Condenses each frame independently by mapping similar pixels within a frame. For example, two shades of red will be merged. This results in a reduction in image quality, but enables the file to be edited in its original form.
Progressive Encoding - Refers to any format where the user is required to download the entire video before they are allowed to watch it.
Internet Streaming - Enables the viewer to watch sections of video without downloading the entire thing, allowing users to evaluate video content after just a few seconds. Quality is significantly lower than progressive formats due to compression being used.
The market for vector graphics has grown considerably, in part, as a result of improved processing and rendering capabilities of modern hardware. Vector-based images consist of multiple objects (lines, ellipses, polygons, and other shapes) constructed through a sequence of commands or mathematical statements to plot lines and shapes in a two-dimensional or three-dimensional space. For Internet usage, this enables graphics to be resized to ever increasing screen resolutions without concern that an image will become 'jaggy' or unrecognisable.
Several vector formats exist for use on the Internet. These construct information in the same way yet provide different functionality. The table below provides a breakdown of the main formats.
| Name | Developer | Availability | Viewers | Uses |
| Scalable Vector Graphics (SVG) | W3C | Open standard | Internet browser | Internet-based graphics |
| Shockwave/Flash | Macromedia | Proprietary | Flash plugin for browser | Video media and multimedia presentation |
| Vector Markup Language (VML) | W3C | Open standard | MS Office, Internet Explorer, etc. | XML-based format. |
For Internet delivery of static images, the W3 recommend SVG as a standard open format for vector diagrams. VML is also common, being the XML language exported by Microsoft products. For text-based vector files, such as SVG and VML, the user is recommended to save content in Unicode.
If the vector graphics are to be integrated into a multimedia presentation or animation, Shockwave and Flash offer significant benefits, enabling vector animation to be combined with audio.
A major feature of vector graphics is its ability to construct detailed objects that can be resized without quality loss. XML (Extensible Markup Language) syntax the basis of the SVG and VML languages is understandable by non-technical users who wish to understand the object being constructed. The example below demonstrates the ability to create shapes using a few commands. The circle, shown on the left, was created by the textual data on the right.
Although XML enables the creation of a diversity of data types it is extremely meticulous regarding syntax usage. To remain consistent throughout multiple documents and avoid future problems, several conventions are recommended:
The use of XML enables a high level of interoperability between formats. When converting for a target audience, the designer has two options:
At the start of development it may help to ask your team the following questions: