Home > Articles > Mobile Application Development & Programming

This chapter is from the book

Understanding Digital Media

These days it’s easy to take digital media for granted. We buy songs and albums from iTunes, stream movies and TV shows from Netflix and Hulu, and share digital photos by email, text, and on the Web. Using digital media has become second nature for most of us, but have you ever given much thought to how that media became digital in the first place? We clearly live in a digital age, but we still inhabit an analog world. Every sight that we see and every sound that we hear is delivered to us as an analog signal. The inner structures of our eyes and ears convert these signals into electrical impulses that our brains perceive as sight and sound. Signals in the real world are continuous, constantly varying in frequency and intensity, whereas signals in the digital world are discrete, having a state of either 1 or 0. In order to translate an analog signal into a form that we can store and transmit digitally, we use an analog-to-digital conversion process called sampling.

Digital Media Sampling

There are two primary types of sampling used when digitizing media. The first is called temporal sampling, which enables us to capture variations in a signal over time. For instance, when you record a voice memo on your iPhone, the continuous variations in the pitch and volume of your voice are being captured over the duration of your recording. The second type of sampling is called spatial sampling and is used when digitizing photographs or other visual media. Spatial sampling involves capturing the luminance (light) and chrominance (color) in an image at some degree of resolution in order to create the resulting digital image’s pixel data. When digitizing video, both forms of sampling are used because a video signal varies both spatially and temporally.

Fortunately, you don’t need to have a deep understanding of the complex digital signal processing involved in these sampling processes, because it is handled by the hardware components that perform the analog-to-digital conversion. However, failing to have a basic understanding of these processes and the storage formats of the digital media they produce will limit your ability to utilize some of AV Foundation’s more advanced and interesting capabilities. To get a general understanding of the sampling process, let’s take a look at the steps involved in sampling audio.

Understanding Audio Sampling

When you hear the sound of someone’s voice, the honking of a horn, or the strum of a guitar, what you are really hearing are vibrations transmitted through sound waves over some medium. For instance, when you strum a G chord on a guitar, as the guitar pick strikes the strings, it causes each string to vibrate at a certain frequency and amplitude. The speed or frequency at which the string vibrates back and forth determines its pitch, with low notes producing low, slow-modulating frequencies and high notes producing high, fast-modulating frequencies. The amplitude measures the relative magnitude of the frequency, which roughly correlates to the volume you hear. On a stringed instrument such as a guitar, you can actually see both the frequency and amplitude attributes of the signal when you pluck the string. This vibration causes the surrounding air molecules to move, which in turn push against their neighboring molecules, which push against their neighbors, and so on, continuously transmitting the energy from the initial vibration outward in all directions. As these waves reach your ear, they cause your eardrum to vibrate at the same frequency and amplitude. These vibrations are transmitted to the cochlea in your inner ear, where they are converted into electrical impulses sent to your brain, causing you to think, “I’m hearing a G chord!”

When we record a voice, an acoustic instrument such as a piano or a guitar, or capture other environmental sounds, we use a microphone. A microphone is a transducer that translates mechanical energy (a sound wave) into electrical energy (voltage). A variety of different microphone types are in use, but I’ll discuss this in terms of one called a dynamic microphone. Figure 1.2 shows a high-level view of the internals of a dynamic microphone.

Figure 1.2

Figure 1.2 Internal view of a dynamic microphone

Contained inside the head case, which is the part you speak into, is a thin membrane called a diaphragm. The diaphragm is connected to a coil of wire wrapped around a magnet. When you speak into the microphone, the diaphragm vibrates in relationship to the sound waves it senses. This in turn vibrates the coil of wire, causing a current to be generated relative to the frequency and amplitude of the input signal. Using an oscilloscope, we can see the oscillations of this current, as shown in Figure 1.3.

Figure 1.3

Figure 1.3 Audio signal voltage

Returning to the topic of sampling, how do we convert this continuous signal into its discrete form? Let’s drill in a bit further into the essential element in an audio signal. Using a tone generator, I created two different tones producing the sine waves shown in Figure 1.4.

Figure 1.4

Figure 1.4 Sine waves at 1Hz (left) and 5Hz (right)

We’re interested in two aspects of this signal. The first is the amplitude, which indicates the magnitude of the voltage or relative strength of the signal. This can be represented on a variety of scales, but is commonly normalized to a range of –1.0f to 1.0f. The other interesting aspect of this signal is its frequency. The frequency of the signal is measured in hertz (Hz), which indicates how many complete cycles occur in the period of one second. The image on the left in Figure 1.4 shows an audio signal cycling at 1Hz and the one on the right shows a 5Hz signal. Humans have an audible frequency range of 20Hz–20kHz (20,000 Hz), so both signals would be inaudible, but they make for easier illustration.

Digitizing audio involves a method of encoding called linear pulse-code modulation, more commonly referred to as Linear PCM or LPCM. This process samples or measures the amplitude of an audio signal at a fixed, periodic rate called the sampling rate. Figure 1.5 shows taking seven samples of this signal over the period of 1 second and the resulting digital representation of the signal.

Figure 1.5

Figure 1.5 Low sampling rate

Clearly, at a low sampling rate the digital version of this signal bears little resemblance to the original. Playing this digital audio would result in little more than clicks and pops. The problem with the sampling shown in Figure 1.5 is that it isn’t sampling frequently enough to accurately capture the signal. Let’s try this again in Figure 1.6, but this time we’ll increase the sampling rate.

Figure 1.6

Figure 1.6 Higher sampling rate

This is certainly an improvement, but still not a very accurate representation of the signal. However, what you can surmise from this example is if you continue to increase the frequency of the sample rate, we should be able to produce a digital representation that fairly accurately mirrors the original source. Given the limitations of hardware, we may not be able to produce an exact replica, but is there a sample rate that can produce a digital representation that is good enough? The answer is yes, and it’s called the Nyquist rate. Harry Nyquist was an engineer working for Bell Labs in the 1930s who discovered that to accurately capture a particular frequency, you need to sample at a rate of at least twice the rate of the highest frequency. For instance, if the highest frequency in the audio material you wanted to capture is 10kHz, you need a sample rate of at least 20kHz to provide an accurate digital representation. CD-quality audio uses a sampling rate of 44.1kHz, which means that it can capture a maximum frequency of 22.05kHz, which is just above 20kHz upper bound of human hearing. A sampling rate of 44.1kHz may not capture the complete frequency range contained in the source material, meaning your dog may be upset by the recording because it doesn’t capture the nuances of the Abbey Road sessions, but for us human beings, it sounds pristine.

In addition to the sampling rate, another important aspect of digital audio sampling is how accurately we can capture each audio sample. The amplitude is measured on a linear scale, hence the term Linear PCM. The number of bits used to store the sample value defines the number of discrete steps available on this linear scale and is referred to as the audio’s bit depth. Assigning too few bits results in considerable rounding or quantizing of each sample, leading to noise and distortion in the digital audio signal. Using a bit depth of 8 would provide 256 discrete levels of quantization. This may be sufficient for some audio material, but it isn’t high enough for most audio content. CD-quality audio has a bit depth of 16, resulting in 65,536 discrete levels, and in professional audio recording environments bit depths of 24 or higher are used.

When we digitize a signal, we are left with its raw, uncompressed digital representation. This is the media’s purest digital form, but it requires significant storage space. For instance, a 44.1kHz, 16-bit LPCM audio file takes about 10MB per stereo minute. To digitize a 12-song album with the average song length of 5 minutes would take approximately 600MB of storage. Even with the vast amounts of storage and bandwidth we have today, that is still pretty large. We can see that uncompressed digital audio requires significant amounts of storage, but what about uncompressed video? Let’s take a look at the elements of a digital video to see if we can determine the amount of storage space it requires.

Video is composed of a sequence of images called frames. Each frame captures a scene for a point in time within the video’s timeline. To create the illusion of motion, we need to see a certain number of frames played in fast succession. The number of frames displayed in one second is called video’s frame rate and is measured in frames per second (FPS). Some of the most common frame rates are 24FPS, 25FPS, and 30FPS.

To understand the storage requirements for uncompressed video content, we first need to determine how big each individual frame would be. A variety of common video sizes exist, but these days they usually have an aspect ratio of 16:9, meaning there are 16 horizontal pixels for every 9 vertical pixels. The two most common sizes of this aspect ratio are 1280 × 720 and 1920 × 1080. What about the pixels themselves? If we were to represent each pixel in the RGB color space using 8 bits, that means we’d have 8 bits for red, 8 bits for green, and 8 bits for blue, or 24 bits. With all the inputs gathered, let’s perform some calculations. Table 1.1 shows the storage requirements for uncompressed video at 30FPS at the two most common resolutions.

Table 1.1 Uncompressed Video Storage Requirements

Color

Resolution

Frame Rate

MB/sec

GB/hour

24-bit

1280 × 720

30FPS

79MB/sec

278GB/hr

24-bit

1920 × 1080

30FPS

178MB/sec

625GB/hr

Houston, we have a problem. Clearly, as a storage and transmission format, this would be untenable. A decade from now these sizes may seem trivial, but today this isn’t feasible for most uses. Because this isn’t a reasonable way to store and transfer video in most cases, we need to find way to reduce this size. This brings us to the topic of compression.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020