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Contents

• Introduction
• Research
  • Direct Sound Capturing
  • Monitoring Skype Conversations
• Implementation
  • Deviare Python Wrappers
  • Wave Tools
  • COM Type Libraries
  • Virtual Table Finder
  • Hooking Direct Sound
• Running Sound Capture
  • Easy Steps
  • Registration
• What’s next
  • Optimizations
  • Wave API Hooking
  • Hook DirectSoundCapture And Listen To Full Conversations
  • Inspect More COM Interfaces

Introduction

Today we are going to see how easy it can be to capture audio with Deviare. From players like Windows Media Player, instant messaging applications like Skype & Windows Live Messenger, to any application using DirectSound. The wave output will get captured by us. Deviare is indeed a powerful framework. Built to resolve most complex tasks in the simplest way. With a few lines of python, all our hooking is done and running. Today performance is extremely important, yet Deviare proves itself as the best. It allows you to also take advantage of the full power of Python

Research

Direct Sound Capturing

I must be honest, I’ve never used the DirectX API in my life, so I was a bit uncertain of how difficult this could be. I started by looking at MSDN documentation on IDirectSound and IDirectSoundBuffer. The first goal was to find a safe place to read its sound buffers. I found out that IDirectSoundBuffer::Unlock could serve my intentions well. At this point, the user is telling DirectSound that he has finished writing his wave output and the locks may be released. So, if we step in between, we can safely read the buffer. The user is no longer writing to it, and DirectSound has not yet taken control of it. I tested it on many applications and it turned to be the right choice. It works perfectly for WMP, Windows Live Messenger, and many others. No problems showed up until I stepped with Skype…

Monitoring Skype Conversations

This might be the way many applications handle their sound output, but it was the first application I have seen and I named the case after it. Later, I found a few articles describing it in detail. So, to my surprise, I was not seeing any data being written to the sound buffers after the unlock was called. How the hell is it writing its wave, and how am I supposed to read it?!. It kept me thinking for a while, until I noticed an interesting and constant call to IDirectSoundBuffer::GetCurrentPosition. Then I realized that this writing method depends on constant reading of the play and write buffer pointers. That’s because DirectSound, as most stream based implementations, works with a Circular buffer. Capturing its wave output requires that we keep track of changes in the write pointer. Once we know it has moved forward in the buffer, we can read its steps. Since here we don’t know how much the user has actually written to it, we must know the full size and location of the buffer. Unless we want to read garbage, but I’m sure that’s not the case .

Implementation

Deviare Python wrappers

Before we get our hands dirty, let me introduce you to something new in Deviare: Python Wrappers. As you already know Deviare is exposed through a series of COM interfaces. To save ourselves from the work of writing a whole new set of bindings, we used the well known project PyWin32. It’s very friendly to be used directly, as you may see in py_deviare_objects.py, just not enough to me. So I built these wrappers on top of it and made them as transparent as possible. You’ll find the use of the interface very similar to the way it’s done in our C# examples and in compliance with the python way of life, of course.

Wave Tools

I wrote these tools to help me write down the captured wave data. This may be obvious to people working in audio projects, but for me I cannot believe there is no native support in Windows to read-write Wav files! Yes, there is native support to write RICH content but come on! Luckily for me, I found a small sample C++ class inside the DirectX SDK. This was good enough for me to write my own in Python. As you may see, my WaveFile class only supports the write operation. Though, adding a read member should be easy enough for you . I have also added a lock to it, to ensure our data does not get corrupted by multiple thread operations. You may use it safely. The structures used were defined exactly as found in DirectSound and WinMM headers. Some of them are used by DirectX to specify the format of the wave content.

COM Type Libraries

By default, DirectX installations do not register their library types. Since we need that information, so Deviare can hook them, I created my own definitions with the interfaces we are interested in. To prevent any collision with previous installations, I used a different GUID. There is a python script that takes care of its registration and it’s automatically ran on demand by our example. Again, definitions are exactly as found on DirectX SDK.

Virtual Table Finder

To obtain the virtual tables for the interfaces, we basically have two options. Either we wait for its instantiation by the target process, or we find them ourselves on our own. Our first option is known to work for sure, yet we delay our installations until these events rise. This may also place us in a race and we may not capture all the output. The second one allows us to hook our targets immediately. Yet, in this case, we depend on the library (dsound.dll) being loaded in the same address space of our target. I have placed the two options in our example. If the current one is not working for you, uncomment the other at py_deviare_directsound.py

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Hooking Direct Sound

The first thing we need, is to know every time a sound buffer is created. For that we are going to intercept calls to IDirectSound::CreateSoundBuffer. If the calls succeed, we look-up the table location inside the returned instance. From there we are going to hook four members of IDirectSoundBuffer:Initialize, SetFormat, Unlock and GetCurrentPosition. The first two, are used to obtain the wave format that the user is writing to the buffer. We also need to watch details from the call that creates the sound buffer, in case it is specified there. The Unlock member, as our research told us, is used in most applications to notify DirectX that the buffer is written and ready to be played. So we read the buffer pointers and size, to use Deviare’s memory interface to copy all content. We need to be careful, and see if the call actually succeeded. Only then can we save the wave data, else we must discard our buffers. With applications that keep track of the play and write cursors, we are going to monitor their calls to GetCurrentPosition. As explained earlier, with this method, I need to know the full size of the buffer and its location. So I save it from the first call to Unlock. Then I virtually divided my buffer in N segments, and filled it with the wave data as the write cursor moves forward in the buffer. Once my buffer contains enough data, I write it down to the wave file. To prevent false positives, in the creation of sound buffers, I delay the creation of my file until I have real data stored. In case we are monitoring the creation of IDirectSound in the target process, we also need to hook DirectSoundCreate and DirectSoundCreate8 from dsound.dll. There we can obtain the virtual table for IDirectSound, and follow our quest.

Running Sound Capture

Easy Steps

Execute the run_me.py located among the deployed files, and you’ll be prompted with a window to type the complete name of the process you want to start monitoring. For example: Skype.exe. Once the program starts capturing, the wave files will be written in the same folder. Once you are done, click OK on the dialog box to stop recording. Now you can open the .wav files generated, and listen the capture. Do not open them before closing the example as the data may not be readable by then.

Registration

The first execution of our example, will automatically register its interfaces and data types. It will also generate a file labeled .deviare_types_registered to prevent registration on the following executions. You can safely remove the file at any time you want the registration to be run again.

What’s Next

Optimizations

At any point of our handling, performance is essential. Any delay is highly punished by the sound output. So we must be careful about any operation we do inside the function call. This example tries to cache enough data before doing a write operation to disk. In case you need to improve its performance, you should read the data and release the call as soon as possible. Then in a different thread, or in a non punitive call, flush our data to the wave file.

Wave API hooking

This example could be very easily adapted to capture wave data from applications using WinMM API. Most browsers, Flash, and Google Talk use it to throw their sound output.

Hook DirectSoundCapture And Listen To Full Conversations

You should have noticed, when capturing from Skype, that your own voice is not heard. That’s because the application is not echoing its capture from the microphone. To get that too, it is necessary to hook IDirectSoundCaptureBuffer and proceed the same way to read its buffer.

Inspect More COM Interfaces

If you want to discover a lot more about the internals of DirectSound, then Deviare will be very valuable for you. Inspecting COM object is very easy indeed. Simply define one of its interfaces (if its not already registered in the system) and hook them the simpler way. If you are wondering what other interfaces may be useful for you, try our Deviare COM Console to discover them. It comes with source code, and you are free to adapt it to your needs! And That’s All Folks, hope you find it useful, enjoy!

Audio Recorder SDK

We did a large number of solutions around this code. We added support to other applications like GoogleTalk, latests versions of Skype and MSN Messenger.

We know how to lead with frequency conversion when mixing the microphone and the audio. Mixing inputs with different sample rate is complex. Nektra can solve this issues for you.

Using all this knowledge together we have developed a complete Audio Recorder SDK that provides a simple API to record full conversations (both microphone and audio together).

Any doubt just ask and we will provide a fast quote for your project.
More information in Nektra Services

See Also

1. Windows API Hooking in Python with Deviare