Windows Phone 8 apps backward compatiability - windows-phone-7

Here is a little question bothering me around, everybody is asking about will wp7 apps run on wp8? but my question is vice-versa. Apps built using sdk8 for wp8 will run on all previous windows phone mobiles especially on wp7.

No, SDK8 uses some Windows phone 8 specific APIs, so Windows phone 7, 7.5, 7.8 users won't be able to download you app.

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FFmpeg for Windows Phone development

I have used FFmpeg in ASP.Net web-based applications. Now I want to use it for Windows Phone development.
Is it possible to use it? If yes, then how?
FFmpeg is not supported on Windows Phone 7 at all and, as far as I could find it on the web, there is no port to Windows Phone 8. Windows Phone 7 never supported native code which was introduced back in Windows Phone 8.
Apparently it is not possible to compile it for Windows Phone 8 and heavy investment must be made.
If you want to play video files, you can do it with the MediaElement control. You can read more about it here: http://developer.nokia.com/Community/Wiki/Video_Playback_with_MediaElement_in_Windows_Phone.

Compatibility between Windows Phone 7.1 - 7.8 apps and Windows Phone 8 devices

Do Windows Phone 7.1 - 7.8 apps run reasonably well on Windows Phone 8 devices?
Is there any reason in terms of compatibility for me to choose one over the other?
You will find the official documentation about WP 7.1 and WP 8 platform compatibility in the following link:
App platform compatibility for Windows Phone
Although in general any WP7 app will run without problem in a WP 8 device, this documentation addresses some cases where the behavior may be different.
So, if you don't use any specific features introduced in WP 8, you can develop targetting WP 7.1. That way your app will be available for more people. If you want to take advantage of WP 8 specific features such as NFC or Nokia maps, I would recommend to develop for both platforms, reusing as much code as you can.
You can run Windows phone 7 apps on a device with Windows phone 8, but not vice versa. I'd advocate targeting 7.x, unless you need to use 8.x specific hardware like NFC, in which case, I'd advocate targeting specific versions.

Can Mono port non xna phone app to windows 8 phone

I'm planning on making a soundboard for windows phone 7 and then hoping to port to windows phone 8. I've had plenty of experience with XNA for game creation but have never made a non game app. It seems easier to do it as a normal phone app using Expression Blend so i can take advantage of using xml.
The question is can you use Mono to port a normal windows phone 7 app this or would it be simpler to just use XNA and port to windows phone 8?
Thanks in advance.
You really don't need to port a Windows Phone 7 app to Windows Phone 8. If you want to add Windows Phone 8 specific features you can do something similar to what is described in this article.

Are windows phone 7 apps compatible on windows phone 8 devices?

very straight forward question. My company has developed a windows phone 7 app and have been testing it on a windows phone 7 device. Can we safely assume that the same app will be backwards compatible and work seamlessly on a windows phone 8 device such as a Nokia Lumia 920?
Thanks in advance!
Yes it will work. But it won't take advantage of all the WP8 features like a real WP8 app would. I'd suggest that you create a copy of the project and update it to WP8 for a better performence. Also replace any listboxes with LongListSelectors for example.
In most scenarios it should just work, but you should check this list of known breaking changes and you should also test your app on new devices thoroughly, just in case.
For example FMRadio class is no longer working on WP8 and there are other similar issues:
App platform compatibility for Windows Phone

How to use phone gap in windows phone 7 mobile application development?

I am iphone and android developer.I know how to use phone gap for iphone app and android app development.But I am new for windows phone 7 app development using phonegap.I dont know how to start windows phone 7 application development using phone gap.
Please give me advice for this query.
Thanks in advance.
Looks like PhoneGap doesn't support Windows Phone yet.
This post seems to suggest its close though and there is an alpha/beta you can start developing
http://programmersgoodies.com/phonegap-for-windows-phone-7
PhoneGap support for Windows Phone 7 is still in a "pre-alpha" state.
From a practical stand it's not realistic to start developing with it. Some people have started (and apparently submitted apps based on it) but there is no guarantee of future compatibility with what they've done.
When available, it will also only support phones running Mango.

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