AmigaOS Manual: Workbench Monitors

There are many different types of monitors that you can use with your Amiga. Because of the numerous options involved, it can sometimes be difficult to find the optimum graphics configuration for the software and monitor you are using. This chapter describes the options and provides additional information about monitors and video modes that you need to know to take advantage of your Amiga's superior graphics capability. In particular this chapter includes information about:

• Your video system
• Monitor drivers
• Choosing a monitor
• Scan rates
• Screen display modes
• Screen centering and overscan
• Working with the AGA chip set
• Mode promotion

Your Video System

Your video system consists of one hardware and two software elements that work together to produce the image you see on the screen. These are:

• The monitor.
• The monitor driver, which is the Amiga software that manages the basic communication between the monitor and the computer.
• The display mode - a set of specific parameters supplied by driver - that controls the actual characteristics of an Amiga screen. These characteristics include the number of displayable colors, the available scan rates, and resolution.

The hierarchical relationship among these elements is illustrated in Figure 7-1.

Video System Hierarchy

For each type of monitor, you can select more than one driver and, within a driver, more than one display mode. You must, however, ensure that your selections are compatible. We strongly recommend that you read this chapter through completely before setting up or purchasing a monitor to use with your new Amiga.

Monitor Guidelines

When setting up a monitor to work with your Amiga, follow these three general monitor guidelines:

• Choose a monitor that suits your needs and is compatible with the Amiga monitor drivers. Check your monitor's documentation for the horizontal and vertical scan rate specifications and compare these to the scan rates listed starting on page 7-6.

Scan rates are listed in kilohertz (kHz). The numbers represent the speed at which the monitor updates the screen display. In general, higher scan rates produce a higher resolution screen with a more stable display.

• When you have determined the kind of monitor you will be using, choose a monitor driver that provides the display modes your particular monitor can display.

Available monitor drivers are present in the Devs/Monitors drawer. Monitor drivers that are not in DEVS:Monitors can be moved three from the Storage drawer. Activate a monitor driver by double-clicking on it. See Chapter 4 for more information on making monitors available.

• When you know which monitor drivers you can use, you can select a display mode. The possible display modes are described and listed starting on page 7-7. Depending on your Amiga chip set, you can have a number of display modes from which to choose. Before settling on a particular display mode, try the different modes available for the monitor driver.

Note that the activation of a driver and the selection of the display mode is accomplished in one step - when you choose a display mode in the ScreenMode Preferences editor.

Some display modes within that monitor driver can appear distorted even though your monitor matches a particular driver. If you choose an incompatible display mode, you should try others until you find one that is clear. Leaving your monitor in an incompatible display mode can cause serious damage to it. If you save a setting before you are sure it is compatible and your display becomes distorted, you can recover using the procedure described on page 7-15. For more information on using the ScreenMode Preferences editor, see Chapter 5.

Types of Monitor

The Amiga requires an analog RGB (color) monitor or high quality color television. There are three general types of analog RGB monitors that you can use with your Amiga. The difference between them is in the horizontal sync frequencies (also called the scan rates) they accept:

1. Video monitors, such as the Commodore 1084 series, accept only the standard Amiga 15 kHz scan rate (NTSC and PAL display modes); a television is considered a video monitor.
2. VGA-only monitors accept only the 31 kHz VGA scan rate (DBL, Multiscan, and Productivity modes).
3. Multiscan (multiple horizontal sync frequency or multi-sync) monitors, such as the Commodore 1942, 1950 and 1960, accept a range of scan rates.

Choosing a Monitor

There are a few things to consider before you choose a monitor. To determine whether a given monitor works with your Amiga, you must compare the specifications of the monitor you are considering against the scan rates listed starting on page 7-6. If the monitor accepts the scan rate of the monitor driver listed, the monitor should work with the display modes the monitor driver provides.

In practice, it is unusual to find a monitor that can display all of the Amiga's display modes. However, there is really no need to do so. Many modes have similar resolutions and differ only in their scan rates. The large number of video modes with different scan rates allows you the flexibility to use any of various types of monitors that are available to you.

Monitor Drivers

A monitor driver contains the range of parameters that a monitor can support. The display mode is a specific subset of those parameters. The display mode you choose must be compatible with your monitor or your screen display may be distorted.

A display mode is defined by:

• Resolution (horizontal and vertical dimensions in pixels)
• Vertical and horizontal scan rates (the speed at which the monitor updates the display)
• Color palette (the largest number of colors that can be displayed on the screen at one time and the size of the palette from which the displayable colors can be chosen)

To determine if you can use a particular display mode, check the horizontal and vertical scan rates listed in this section against the specifications for your monitor. If the display mode's scan rates are outside your monitor's acceptable range, you cannot use the mode.

When choosing a monitor and monitor driver, you should consider the following:

• Certain important Amiga screens, such as alerts and the Early Startup Control screen, always appear in one of the 15 kHz modes, as do many games and certain other applications. A VGA-only monitor cannot properly display these screens.
• A 15 kHz-only monitor can display all system screens and virtually all Amiga applications, but cannot take advantage of Mode Promotion or the higher resolutions and enhanced displays that the AGA chip set offers.
• Vertical scan rates (also called the refresh rate) for display modes available under Release 3 vary from 48 to 73 Hz.

Check the specifications for a multiscan monitor carefully, however, as not all multiscan monitors accept 15 kHz input.

If you are not using a monitor that scans to 15 kHz, it is important to have your initial system settings made using a monitor that displays 15 kHz modes. Once your initial settings are made and you can use your regular monitor, you should make at least one backup floppy copy of your Workbench software containing these settings. If your ScreenMode editor display settings are accidentally changed, you can use this floppy disk to recover your display. Procedures for recovering a distorted display are described on page 7-15.

Required Scan Rates

The following sections group the monitors drivers according to the three basic horizontal scan rates that they require.

Video Monitors

Monitor Driver	Horizontal Scan Rate	Vertical Scan Rate
NTSC	15.72 kHz	60 Hz
PAL	15.60 kHz	50 Hz
Euro:36Hz	15.76 kHz	73 Hz

VGA Monitors

The standard VGA horizontal scan rate is 31.5 kHz. Most, but not all, VGA-only monitors can also display the 27 kHz promoted modes produced by the DBLNTSC and DBLPAL monitor drivers.

If your are attempting to use a VGA monitor with the DBL or Multiscan modes and it does not display those modes correctly, activating the VGAOnly monitor driver can help. It modifies the scan rates slightly to better match the VGA rates. However, this does cause the left edge of the screen to flash when scrolling horizontally, so you should not use VGAOnly unless it is necessary.

The next table shows the normal scan rate followed by the rates as modified by VGAOnly. If you activate or deactivate VGAOnly, you should delete the file ENVARC:Sys/overscan.prefs and reboot before editing your ScreenMode and Overscan Preferences settings.

Monitor Driver	Horizontal Scan Rates Normal/VGAOnly	Vertical Scan Rates Normal/VGAOnly
DBLNTSC	27.66 kHz / 29.30 kHz	58 Hz / 59 Hz
DBLPAL	27.50 kHz / 29.45 kHz	48 Hz /50 Hz
Euro:72Hz	29.32 kHz / 31.43 kHz	69 Hz / 70 Hz
Multiscan	29.29 kHz / 31.44 kHz	58 Hz / 60 Hz

Multiscan Monitors

A multiscan monitor can potentially display any of the previously listed rates, plus the following:

Monitor Driver	Horizontal Scan Rates	Vertical Scan Rates
Super72	23.21 kHz / 24.62 kHz	71 Hz / 72 Hz